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THE KILN DRYING 
OF LUMBER 

A PRACTICAL AND THEORETICAL 
TREATISE 



BY 

HARRY DONALD TIEMANN, M.E., M.F. 

IN CHARGE, SECTION OF TIMBER PHYSICS AND THE KILN DRYING EXPERIMENTS OF THE 

U. S. FOREST SERVICE. SPECIAL LECTURER IN WOOD TECHNOLOGY AND 

FORESTRY, UNIVERSITY OF WISCONSIN. FORESTRY PRODUCTS 

LABORATORY, MADISON, WISCONSIN 



ILLUSTRATED 




i, . 

PHILADELPHIA AND LONDON 
J. B. LIPPINCOTT COMPANY 






COPYRIGHT, I917. BY J. B. LIPPINCOTT COMPANY 



.- -i 
«»-^'j 



17' 



Electrotyped and Printed hyJ. B. Lippincott Company 
The Washington Square Press, Philadelphia, U. S. A. 



NOV 26 1917 
©CI,A47949S 



^^f^ 



PRECAUTIONARY NOTICE 

Wood wMch has been bent in iron forms should 
not be steamed again after the forms have been re- 
moved, but the drying should be started at once at a 
humidity less than saturation. Bent wood is in a 
peculiar condition due to the internal stresses intro- 
duced in the bending, and if steamed again after it has 
become " set " the fibers will soften up to such an 
extent that the wood will warp out of shape or even 
rupture on the tension side. This caution applies par- 
ticularly to heavy pieces, such as bent oak wagon rims. 
When bent wood, after it is dry, is steamed, it will 
tend to resume its original form with considerable force. 
Bent wood should therefore not be steamed for removal 
of casehardening. 

H. D. TiEMANN. 
October 18, 1917. 



lu 



CONTENTS 



CHAPTER PAGE 

I. Introduction 1 

II. The Structure and Properties op Wood 10 

III. Common Practices in Drying 23 

IV. How Wood Dries; Shrinkage, Warping, Casehardening . . 103 
V. The Principles op Kiln Drying 137 

VI. The Circulation and the Method op Piling 154 

VII. Special Problems in Drying 180 

VIII. The Improved Water Spray Humidity Regulated Dry 

Kiln 191 

IX. Drying by Superheated Steam and at Pressures Other 

Than Atmospheric 200 

X. Theoretical Considerations and Calculations, Humidity, 
Evaporation, Density, the Drying Cycle, Amount op 

Air and Heat Required, Thermal Efficiency 216 

XI. Effect op Different Methods op Drying Upon the 

Strength and the Hygroscopicity op Wood 256 

XII. Instruments Useful in Dry Kiln Work and Methods of 

Testing Wood 265 

XIII. Temperatures and Humidities for Drying Various Kinds 

OF Lumber 272 

XIV. Humidity Diagram 286 

Appendix 300 






ILLUSTRATIONS 

FIG. PAGE 

1. Row containing five hundred thousand board feet of Douglas fir 

and Western larch 2 

2. Highly magnified block of wood (hardwood) 12 

3. Comparative sizes of the wood elements of red gum 14 

4. Highly magnified portions of the elements of red gmn wood 15 

5. Cross section of red gmn wood, showing the structure 15 

6. Cross section of red oak highly magnified 18« 

7. Cross section of yellow pine highly magnified 19 

8. Air-dried and kiln-dried oaks, showing honeycombing 24 . 

9. Progressive veneer drier 32 

10. Heated plate veneer drier 33 

11. Typical progressive ventilated type of kihi 38 > 

12. Type of forced draught dry kiln 42 

13. Cross section of a ventilated type of kiln 43' 

14. Flat piling 49' 

15. Hussey door carrier 80 

15a. Double canvas doors 80 

16. Black walnut gim stock blanks arranged with inclined piUng for 

kiln di'ying 82 

17. The disastrous effect of improper methods of kiln drying black 

walnut gun stock blanks 83 

18. Greneral layout of plant with progressive kilns 101 

19. General layout of plant wdth compartment kilns 101 

20. Relationship between relative hmnidity of the air and percentage 

of moisture retained by cotton and by several species of 

wood 105 

21. First stages in casehardening 115 

22. Final stage in casehardening and honeycombing 117 

23. End view of a casehardened board 118 

24. Test dis!:f5 for removal of casehardening 120 

25. Reversal of casehardening shown by disks 124 

26. Diagram explanatory of spiral grain 125 

27. How wood shrinks 126 

vii 



viii ILLUSTRATIONS 

28. "Washboarding" effect on radially-sawed board of blue gum 127 

29. Remarkable shrinkage of California blue gum compared with 

redwood 127 ' 

30. Actual drying curves obtained in kiln drjdng green one-inch red 

gum lumber 146 

31. Air drying curves for two pieces of green western larch sap wood 150 

32. Flat crosswise piling effective when air currents in kiln are in the 

horizontal longitudinal direction 158 

33. Cross section of kiln 159 

34. Cross section of kiln 160 

35. Cross section of kiln 161 

36. Edge-piled lumber 171 

37. IncHned pile of boards, sloped in wrong direction 172 

38. Inclined pile of boards correctly sloped 173 

39. Case in which the condenser is so placed as to oppose the natural 

circulation and cause stagnation in part of the pile 174 

40. Downward circulation in a ventilated kiln observed by tempera- 

ture measurements and pressure determinations 175 

41. Circulation as observed in a pile of cold lumber, shortly after be- 

ing placed in the kihi 17S 

42. Collapse in western red cedar boards 184 

43. Magnified sections of western red cedar boards, showing the 

beginning of the "Explosive" effect 185 

44. Diagram of the Tiemann water spray humidity regulated dry kiln 192 

45. Arrangement for flat piling 195 

46. Diagrammatic sketch of water spray and condenser system 197 

47. Recording thermometer record of temperatxire of kiln thermostati- 

cally controlled 202 

48. Simple arrangement for use of high velocity superheated steam 

method with incUned piUng system 203 

49. Arrangement for use of high velocity superheated steam method 

with edge pihng system 204 

50. Arrangement for use of the new high velocity low superheat 

method with flat piled Imnber and with reversible circulation 205 

51. Block of green red oak after removing from steaming at 212° to 

225° for eight hours 212 

52. Diagrammatic plan of drying cycle 237 



ILLUSTEATIONS jx 

53. Curves for an actual drying run on oak, hickory, and birch wagon 

stock 275 

54. Humidity regulated experimental kilns at the Forest Products 

Laboratory 286 

PLATES 

I. Drying conditions suitable for one-inch red gum, black gum, 

and black walnut 279 

II. Drying conditions suitable for one-inch select sap head maple 

and select basswood 280 

III. Drying conditions suitable for one-inch yellow birch, ash and 

chestnut for ordinary purposes 281 

IV. Drying conditions suitable for black walnut, red and white oaks, 

and other dense hardwoods and for mahogany and black 
walnut 282 

v. Drying conditions suitable for one-inch western larch and 

cypress 283 

VI. Drying conditions suitable for western red cedar and redwood 

"sinker" stock 284 

VII. Drying conditions suitable for Douglas fir, yellow pines, incense 

cedar, and many other softwoods 285 

VIII. Hmnidity diagram 286 



TABLES 



PAGE 

I. Consumption of wood in cubic feet per capita in various coun- 
tries before the war 2 

II. Mineral content in 1000 parts by weight of dry wood substance . . 21 

III. Effect of soaking in fresh and salt water upon the shrinkage of 

wood 27 

IV. Drying by the high velocity superheated steam method 50 

V. Flow of steam in pounds per minute through a straight pipe 100 

feet in length with a reduction of pressure of 1 pound per 

square inch 66 

VI. The fiber saturation point for several woods as determined by 

compression* tests on small specimens 104 

VII. Shrinkage from green to oven-dry condition 129 

VIII. Comparison of temperatures inside and outside a stick of wet 

wood placed in a dry kiln 167 

IX. Maximum possible theoretical heat efficiency of evaporation 

under given conditions at atmospheric pressure 245 

X. Increase in density of air due to spontaneous cooling produced 

by evaporation 253 

XI. Weakening effect of various processes of drying on the strength 

of white ash 259 

XII. Weakening effect of various processes of drjdng on the strength 

of loblolly pine 260 

XIII. Weakening effect of various processes of drying on the strength 

of red oak 261 

XIV. Changes in hygroscopicity produced by various processes .... 263 
XV. Shrinkage of plain sawed boards from a kiln-dried condition 
of 5 per cent, moisture to total dryness, in inches per 
inches of width 271 

XVI. List of species for drying curves 278 



XL 



THE 
KILN DRYING OF LUMBER 

CHAPTER I 

Intkoduction 
the objects of deying wood and the need op a 

TECHNICAL KNOWLEDGE OF KHjN DRYING 

"Wood, next to stone, is doubtless the earliest ma- 
terial used by man and has served bis needs more 
than any other substance. In fact, it is safe to say 
that civilization could not exist were it not for the 
forests and the v/ood derived therefrom. Kingdoms 
have risen and fallen and become extinct, as the forests 
have flourished, been wantonly destroyed and perished. 
In China, for example, in the Shan-Si and in the Chi-Li 
Provinces, and near Fou-Ping, vast regions, once 
thickly populated, are now desolate wastes or wholly 
uninhabitable through the prodigal destruction of their 
greatest source of prosperity — the forests which once 
clothed the steep mountains, preventing floods and 
erosion, the washing away of the fertile soils, and 
which furnished fuel and building material in abun- 
dance. In the Southern Alps and in the Pyrenees Moun- 
tains, in Europe and Greece, and in Northern Syria 



2 THE KILN DRYING OF LUMBER 

and Palestine, and in Northern Africa, the same results 
have followed the loss of the forests. Through heroic 
efforts and vast expense France and Italy have been 
reestablishing the habitability of the land by re- 
planting. 

But it is not with the living tree that our present 
subject has to do — all of that belongs to the province 
of Forestry — ^but rather with the product after the tree 
has been cut down and sawed into lumber or other 
forms of wood. Lumbering and Forestry are not an- 
titheses, but both should accomplish the same result, the 
perpetuation of the forest for future use, whatever 
that may be. Lumber is a crop, as are vegetables, 
and the forest may be utilized, and at the same time 
conserved for future use. 

Table I. — Consumption of Wood in Cubic Feet Per Capita in Various 
Countries Before the War ( Including Firewood ) . 

From Forest Service Bull. 83, Forest Resources of the World, by Raphael Zon. 

Cu. Ft. Cu. Ft. 

United States 260.0 Japan 30.0 

Canada 192'.0 France 24.6 

Norway 125.0 Denmark 19.8 

Sweden 120.0 Belgium 17.7 

Finland 91.5 United Kingdom 14.0 

Russia 63.0 Holland 13.1 

Austria-Hungary 57.0 Italy 13.0 

Switzerland 38.0 British India 0.8 

Germany 36.6 

To give an idea numerically of the intimate relation 
which wood bears to every individual, the table above 
is inserted, which shows, before the Grreat War, what 



INTRODUCTION 3 

the relative consumption of wood per capita has been 
in various countries. 

It is evident that wood in the living tree contains 
a great deal of moisture. In fact, the walls of the cells 
of which the wood is composed, as we shall see later 
on, are saturated and never become dry in the natural 
state. Trees contain anywhere from 30 to over 200 
per cent, of the dry weight of the wood in water. This 
moisture renders the wood very heavy and unsuitable 
for use for most purposes. It must be got rid of before 
the wood will burn for fuel, and before lumber is used 
for houses or furniture or any manufactured article. 
For some uses, as piles, concrete forms, etc., wet wood is 
desirable, but in the vast majority of cases it must 
be dry. 

When a tree is cut down and sawed into lumber, 
this will gradually dry in the air, if piled so the air can 
get at it, to a certain degree depending upon the aver- 
age humidity of the climate, and is then called in gen- 
eral terms ''air dry" (Fig. 1). This condition will 
range from 8 to 18 per cent, of water, based on the per- 
fectly dry weight, according to the climate and time of 
year. In the neighborhood of New York City this con- 
dition will average about 14 per cent, during the sum- 
mer time, if protected from the rain and dampness. It 
takes a long time for wood to become air dry, depend- 
ing upon the species, the thickness of the boards, the 



4 THE KILN DRYING OF LUMBER 

initial amount of moisture, and the manner of exposure 
and the climate. One-inch white oak, for instance, will 
require from one and a half to three years to thoroughly- 
air season; white pine, on the other hand, may thor- 
oughly dry in four months. If left in the log it will 
require many years for the water to evaporate, espe- 
cially if the bark remains on. In fact, except for very 
durable species, as cypress or white oak, the wood is 
apt to completely decay before it becomes dry, in the 
log form. 

All wood in drying shrinks and is, therefore, liable 
to warp and check. Moreover, the strength of the wood 
substance greatly increases as it dries below the '* fibre 
saturation point, ' ' which is the name given to the con- 
dition of the wood when the cell walls become saturated 
with moisture (about 25 to 30 per cent, of the dry 
weight). 

Briefly, leaving out the use of wood as fuel, the 
objects of drying wood may be summed up as follows : 

1. To render it suitable for the purpose for which 
it is to be used. 

2. To lessen the weight for handling and shipping 
purposes. 

3. To prevent decay. 

4. To increase its hardness and strength, and to 
prevent subsequent shrinkage and ''working" after 
it has been placed in a building or manufactured article. 



INTRODUCTION 5 

For many purposes, sucli as use in heated buildings, 
cooperage, gun stocks, athletic goods, etc., "air dried" 
wood is not sufficiently dry, and it must be further dried 
by means of heat in a dry kiln. Furthermore, many 
woods, such as red gum, western larch, and southern 
oaks, seldom air dry without great losses in warping, 
checking, case hardening, and honeycombing. These 
losses may be greatly reduced or entirely eliminated 
by properly kiln drying the green material in kilns 
where the humidity and circulation and temperature 
can be maintained at the most suitable amounts. For 
some purposes, as flooring, cabinet work, etc., the 
''working," that is, the twisting of the wood due to 
swelling and shrinking under changes of the atmos- 
phere, may be slightly reduced by thorough kiln 
drying. 

The principal objects, therefore, of kiln drying in- 
stead of the slow process of air drying may be enu- 
merated under the following headings: 

1. To improve the condition of the wood for the 
purpose for which used. 

2. To reduce losses which occur in air drying. 

3. To reduce the time necessary to carry stock in 
the yard. 

4. To reduce shipping weights without waiting for 
the long time required to air dry the material. 

All kinds of methods have been tried and several 



6 THE KILN DRYING OF LUMBER 

hundred patents taken out for apparatus, processes, 
and kilns, for drying wood. Many of these have not 
proved to be a success. The processes include pre- 
liminary treatments of steaming at atmospheric pres- 
sure and at pressures up to forty or even sixty pounds 
gauge, soaking in various solutions, drying in vacuum, 
in compressed air, in alternating compressed air and 
vacuum, in compressed air and steam, in superheated 
steam, and in gases such as waste products of com- 
bustion from chimneys, and even electrical treatments. 
Temperatures vary all the way from 60° F. up to 
300° F. and even 400° F. in gases other than air. 

Apparatus of all kinds have been tried, also, in- 
cluding steel cylinders, accordion-like heaters, ovens, 
and drying rooms of all shapes and sizes. 

While some species, such as pine, fir, basswood and 
mahogany, are easy to dry, many others, as oak, gum, 
and notably eucalyptus (blue gum), are exceedingly 
difficult to dry, requiring totally different treatment. 

A great amount of lumber is being kiln dried chiefly 
for the purpose of reducing shipping costs and for 
eliminating the investment necessary to carry a large 
stock on hand. This is particularly true with respect 
to the "West Coast timbers, especially Douglas fir, and 
also of southern yellow pine. For this purpose the 
quickest possible method of drying consistent with not 
seriously injuring the sales value of the stock is sought 



INTRODUCTION 7 

after, and higli temperatures and superheated steam 
are largely used. 

In 1913 it is estimated that the total cut of lumber 
in the United States (exclusive of poles, posts, hewn 
ties, and firewood) (sawed into lumber or timber at 
mills) was 38,388,110,000 board feet, and 88,984,000 
additional was imported. The various ways in which 
this was used are given below : 

Wood-using factories 24,576,557 M 

Exports of lumber and timber (planks, sawed ties, etc.) . . 3,185,864 M 
Consumed by railroads: 

Sawed ties 1,215,597 M 

Bridge and trestle timbers 841,859 M 

Miscellaneous sawed wood 28,8i04 M 

Building lumber, studding, joists, rafters, and rough con- 
struction 8,628,413 M 

Total less imports 38,477,094 M 

Out of the 24,576,557M used in factories, an estimate 
made according to 54 distinct wood-using industries 
shows that approximately 15,116,000M softwoods and 
4,725,000M hardwoods were kiln dried before using. 
The hardwoods are mostly first air dried and then put 
through the kiln. Many of the softwoods are dried 
directly from the saw. Assuming an average value of 
$16 for softwoods and of $21 for hardwoods, this figures 
out a value of $241,856,000 for softwoods which are 
annually kiln dried and of $99,225,000 for hardwoods. 

The official estimate for the total lumber cut for 
1915 is 37,013,2943^^, which is not greatly different 



8 THE KILN DRYING OF LUMBER 

from that of 1913, so it is fair to assume that the figures 
hold throughout very closely for the present condi- 
tions. If the present losses in preliminary air drying, 
which amount to considerably over 12 per cent, for 
hardwoods and 5 per cent, for softwoods, could be 
reduced to 2 per cent, by the best methods of kiln 
drying direct from the green condition, there is a 
possible annual saving of $17,178,000 alone, not in- 
cluding lumber which is not now being kiln dried. 

The value of a technical knowledge of kiln drying 
is, therefore, self-evident, and it is the purpose of this 
book to give the reader the best information available 
on this important subject. The United States is ac- 
knowledged to be in advance of all other countries in 
this particular phase of the wood problem. 

Thus far, I have touched only upon the losses which 
it is possible to obviate to a large extent. Let us con- 
sider some of the positive advantages to be gained by 
better methods of drying. 

In the first place, it would enable architects, engi- 
neers, and wood-users to specify what they required 
in regard to this condition of the wood with a degree 
of exactness, as is now done with other materials. 
How would it seem for an architect, for instance, to 
order edge grain western larch flooring for his build- 
ing and to state that it should contain between 5 and 
7 per cent, moisture, that it should have been manu- 



INTRODUCTION 9 

factured only when it contained 5 per cent., that it 
should not be casehardened beyond a certain degree 
determinable by a simple test, be free from all checks 
or honeycombing, and that it should not have been 
heated beyond 160° in the dry kiln while it was moist 
in order to avoid brittleness. Who would accept such 
an order to-day? Yet when lumber dealers can come 
up to such standards, which is by no means a Utopian 
idea, there will be little necessity to ^ ^boom" the lumber 
markets, for the customer will get just what he wants 
and be satisfied. 



CHAPTER II 

The Stkucture and Propeeties op "Wood * 
A TECHNICAL discussion of the minute anatomy of 
wood as revealed by the microscope need not be given 
here, as it can be found in botanical works, but some 
idea of the structure and the way in which different 
species differ from one another is deisirable for a 
clear understanding of the manner in which it behaves 
in drying. In general, v/ood is built up of individual 
^'cells'' of various shapes and sizes which have been 
formed by the '' cambium" or the region of active 
growth, consisting of a thin layer of soft, succulent 
tissue lying between the inner bark and the wood 
proper. Every year this cambium deposits a new 
layer of wood upon the outer circumference of the 
central cylinder, thus forming the ''annual rings" or 
the "grain" of the wood. In most trees the several 
last-formed outer layers remain active and conduct 
water and mineral substances from the roots upwards 
to the leaves. This is the "sapwood" of the tree. 
The inner layers of the sapwood, after they have func- 
tioned in this way for a number of years, finally be- 

*For a more complete discussion of the properties of wood, the 
reader is referred to " The Economic Woods of the United States," by 
Samuel J. Record, Wiley & Sons, 1912; also to " Wood," by G. S. Boulger, 
Arnold, 1908. A complete list of references is given in Record's book. 

10 



THE .STRUCTURE AND PROPERTIES OF WOOD 11 

come inactive and are converted successively into lieart- 
wood, which, so far as the living functions of the tree 
are concerned, is dead wood, but it still acts as a 
mechanical support to the tree. Certain chemical 
waste products are deposited within the cell walls from 
the living protoplasm in the sapwood as it becomes 
converted into heartwood, thus giving the usual darker 
color to this portion. In some species, as white beech, 
heartwood is never formed, and in others, as balsam 
fir, no change in color takes place. Several distinct 
kinds of cells are formed by the cambium layer in 
some species of wood, which are as a rule long and 
tapering, some being almost hair-like in shape and 
others much wider in proportion to length. These 
cells are arranged vertically in the tree, but in addi- 
tion they are interspersed by numerous horizontal 
groups of thin-walled, pith-like cells arranged radially 
in the trunk in strands shaped somewhat like two- 
edged swords placed edgewise. These are the *' medul- 
lary rays" (silver grain in quarter-sawed oak), and 
serve to conduct food and moisture from the sapwood 
and the bark to the growing cambium layer. It is on 
account of these medullary rays that wood dries more 
rapidly in the radial direction than in the tangential. 
Nearly all woods may be classed into two principal 
groups, having very marked distinctions, with respect 
to the elements of which they are composed. The so- 



12 THE KILN DRYING OF LUMBER 

called ''hardwoods," or broad-leaved trees, more cor- 
rectly the *' ' angiosperms, " are the more complex in 
structure, whereas the commonly called "softwoods," 
or needle-leaved trees, the conifers, or more correctly 
the "gymnosperms," are comparatively simple. The 
former are called porous woods on account of the 
tubular openings which they contain, known as vessels. 
These vessels are entirely lacking in the gymnosperms. 
In some woods, as red oak, these vessels are completely 
open passages, so that air may readily be blown through 
a stick of the green wood many feet in length. In 
others, such as white oak, for example, the tubes are 
blocked by ingrowths of thin-walled cells known as 
tyloses. It is impossible on this account to force any 
appreciable amount of air through a piece of fresh 
white oak even an inch in length. This is why white 
oak is such an excellent wood for tight cooperage, 
whereas red oak would make a leaky barrel. 

In addition to the vessels, hardwoods (angiosperms) 
contain several distinct types of cells. 

Fig. 2 is a sketch from the actual microscopic views 
showing a small block of one of the simplest of the 
hardwoods, tulip or yellow poplar, as it would appear 
if highly magnified. TT is a horizontal surface cut 
across the grain. EE is a radial surface (quarter- 
sawed) and TG- is a tangential surface (plain sawed). 
AE is one annual ring, S being the first formed layer 




Fig. 2.— Highly magnified block of wood (hardwood). 



THE STRUCTURE AND PROPERTIES OF WOOD 13 

or the springwood composed of large thin-walled cells, 
and SM the later formed layer of small thick-walled 
cells or summerwood. It is the relative thickness of 
the cell walls which gives the density and hardness 
to certain woods, as hickory, compared to thin-walled 
woods, as basswood, and which makes the hard part 
of the rings in yellow pine. MR and MR are medul- 
lary rays. V is a vessel cut in section and showing 
its remarkable end splicing on to the cell below by 
means of a scalariform grating. These grating-splices 
are shown in three places marked SC. They differ in 
form in different woods, in some, as birch, being simple, 
elliptical, or circular openings. The vessels com- 
municate with adjacent vessels by means of numerous 
small partial openings called "bordered pits." All 
cells retain their individual walls, which become thick- 
ened as the cell develops from the cambium layer by 
successive depositions laid down on the inside by the 
living protoplasm. Adjacent walls between two cells 
are, therefore, composed of five distinct layers, the 
original thin primary walls of the two contiguous cells 
and an intermediate substance by which they are joined 
together. The two primary walls, together with the 
cementing substance, are collectively known as the 
"middle lamella." On either side of this "middle 
lamella" are the thickened walls of the two contiguous 
cells. The pits are openings in the thickened walls, 



14 



THE KILN DRYING OF LUMBER 



but do not pass through the middle lamella. While the 
pits serve as communicating regions for the proto- 
plasm, they must not be considered as openings in the 




Fig. 3. — Comparative sizes of the wood elements of red gum. F, wood fiber; D 
duct or vessel (trachea) ; M, medullary ray cells. (Forest Service Bulletin 58. Drawn by 
the author.) 

sense of punctures in the walls. Numbers of these 
''bordered" pits are shown on the walls of the vessels 
in the figure. 



THE .STRUCTURE AND PROPERTIES OF WOOD 15 

The other cells, which in this species are almost all 
''wood fibers" with thick walls and small cavities, are 
what give the rigidity and hardness to the wood. They 
are shown at F and ML. These wood fibers also have 
pits which are very small and slit-like. One is shown 



Fig. 4. 



Fig. 5. 





jNii)rii i Min|i HI II nij Hifi 



Fig. 4. — Highly magnified portions of the elements of red gum wood. D, end of 
a duct or vessel (trachea), showing the peculiar grating; F, pointed end of a fiber; M, 
short medullary ray cells. The narrow, slit-like pits are seen in the walls of each element. 

Fig. 5. — Cross section of red gum wood, showing the structure. F, wood fiber; 
D, duct or vessel (trachea) ; M. medullary ray cells. 

at P. Wood fibers vary in average lengths in different 
species from one to two millimeters. 

Some idea of the relative sizes of the various cells 
in red gum, which is almost identical to tulip in struct- 
ure, is shown in Figs. 3, 4, and 5 (from Forest Service 
Bulletin 58), which are sketches made by the writer 
directly from the microscope. 



16 THE KILN DRYING OF LUMBER 

Oak is considerably more complex in structure than 
the woods described above, but for a more detailed 
description the reader is referred to works on wood 
structure.^ 

The gymnosperms or coniferous "softwoods," on 
the other hand, are much simpler in structure than the 
tulip just described. In the firs, for instance, the verti- 
cal cells are practically all of one kind, called tracheids, 
and are of approximately the same widths in the cir- 
cumferential directions, though differing considerably 
in thickness, measured radially, from the large, thin- 
walled springwood cells to the small, thick-walled sum- 
merwood cells. In section they are nearly rectangular 
and are arranged in very even and uniform radial rows. 
The medullary rays are very small, numerous and 
uniformly arranged. This remarkable simplicity of 
structure gives to this class of woods certain properties 
which are lacking or less strongly marked in the other 
class. They are remarkably stiff in proportion to their 
weight, which renders the conifers especially suitable 
for building and structural purposes. Also, their 
resonance properties are good, so that they are used 
for sounding boards, etc., in musical instruments. In 
the pines especially, and also in spruce and larch, there 
are in addition to the tracheids many tubular openings 
called "resin ducts," in which the resin is stored and 

* " The Oak," by Marshal Ward. 



THE .STRUCTURE AND PROPERTIES OF WOOD 17 

out of which it flows when the tree is injured or tapped. 
These resin ducts are not cells but are openings between 
the cells. The tracheids all have numerous, bordered 
pits. These bordered pits are of peculiar structure, 
deserving notice in considering the drying or impreg- 
nation of wood. The middle lamella extends straight 
across the round opening of the pits of the two con- 
tiguous cells like the membrane of a drum, the open- 
ings being always in exact aligimient. Imagine two 
soup plates placed face to face with their rims 
touching, a membrane stretched across between them 
and round holes cut in the bottom of each, and a very 
good conception will be had of the shape of the bor- 
dered pits. The membrane has a thickened circular 
disk at its center which is larger in diameter than the 
two holes in the ''borders." When wood dries this 
thickened disk, called the "torus," becomes pressed 
to one side tightly against the border, to which it 
firmly adheres, thus effectively sealing the opening 
of the pit. This phenomenon occurs chiefly in the 
springwood of the annual rings, since the pits in the 
summerwood portions do not as a rule become closed 
in this manner. Just how these remarkable structures 
behave in the conduction of water up the tree is not 
known, but they must have an important function in 
this regard, as they are present in all the water-con- 
ducting tissue. 



18 THE KILN DRYING OF LUMBER 

In the pulp and paper industry these tracheids are 
indiscriminately spoken of as ''wood fibers." But the 
true wood fibers belong to the hardwoods and are of 
quite a different form. 

Trees native of tropical countries often do not have 
the annual rings, since their growth is more or less 
continuous, but even in hot countries irregular rings 
are sometimes formed, due to periodic seasons of active 
growth. 

There is also another kind of wood not included 
in the description given above, which occurs in the 
palm trees, bamboo, rattan, sugar cane, etc., in which 
the water-conducting vessels, wood fibers, and inner 
bark occur grouped together in small strands inter- 
spersed all through the stem with pith tissue between. 
The wood is not formed by concentric rings laid down 
circumferentially from a cambium layer, but growth 
takes place by an increase in the numbers of these 
strands. The strands are about the size of broom 
straws and interlace throughout the stem in the form 
of a network. These trees, however, are not com- 
monly used as lumber and need not receive further 
consideration here. They used to be called "endo- 
gens" on account of the way in which the growth of 
the stem took place, and the others with concentric 
layers, "exogens." The endogens belong also under 
the angiosperms, but the seeds are monocotyledonous 




Fig. 6. — Cress section of red oak highly magnitied. V, springwood vessel; S, summer- 
wood vessel; M, M, medullary ray. 




Fig. 7. — Cross section of yellow pine highly magnified. A, springwood tracheida; 
B, summerwood tracheids; D, resin duct; M, medullary ray. 



THE STRUCTURE AND PROPERTIES OF WOOD 19 

(sprouting like corn), whereas the seeds of the exogens 
are dicotyledonous (sprouting like beans). 

Figure 6 is the cross section of a piece of red oak 
showing a large vessel at V, a small one at S, dense 
wood fibers at B and a medullary ray MM. Figure 7 
is a cross section of a piece of pine magnified the same 
amount, showing a resin duct D, summerwood tra- 
cheids B, springwood tracheids A, and medullary 
ray M. 

All wood substance, irrespective of species, weighs 
about the same amount, its specific gravity being 1.56. 
Thus it will sink in water. The buoyancy of wood is 
due to its air spaces, and the lighter the wood the 
thinner are its cell walls. It is also probable that the 
strength of the wood substance itself is nearly the same 
in all species. 

It is supposed that the cell walls are composed of 
minute particles almost molecular in size, and that 
moisture enters between these particles by molecular 
attraction or cohesion. As the amount of moisture 
increases the particles are jDushed apart until their 
mutual attractive force counterbalances the cohesion 
attraction of the moisture, when they will absorb no 
more. This accounts for the hygroscopicity of wood 
and its power to absorb moisture from air which is 
only partly saturated. It is merely a hypothesis, as 
the particles are too small to be seen by any micro- 



20 THE KILN DRYING OF LUMBER 

scope. In this respect wood is a colloidal substance. 
The shape and arrangement of these hypothetical par- 
ticles may account for the way in which wood swells 
differently in different directions. 

Chemically wood substance is exceedingly complex 
and has never been completely determined. It is basic- 
ally the same in all species, but is impregnated with 
different substances. The basic substance is cellulose 
(CeHioOg)^, and with this is combined lignin, forming 
what is called lignocellulose. Lignin, however, has 
never been isolated by itself. Cotton is pure cellulose. 
In the manufacture of sulphite pulp, the lignin is ex- 
tracted and nearly pure cellulose is left. The pro- 
portional composition of cellulose is the same as for 
starch and its component relationship to sugar is seen 
by the equation 

2C8H10O5 4" H2O = Ci2H220ii 

Starch + water = sugar 

The conversion which can be accomplished, how- 
ever, is not quite so simple as the above equation might 
seem to indicate. 

In addition to the wall substance, wood may con- 
tain within the cells, gums, resins, combined acids, 
starches, volatile and essential oils, and very little 
albuminoids. The sapwood, particularly, is rich in 
gums, starches, and sugars. There are also traces of 
many very complex organic compounds present in most 



THE STRUCTURE AND PROPERTIES OF WOOD 



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22 THE KILN DRYING OF LUMBER 

woods and also mineral substances, as potash, calcium, 
magnesium, sulphur, phosphorus, iron, and soda. 

The foregoing Table (II) gives the analyses for min- 
eral content by Dr. Eudolf Weber and by Hartig. In 
using a table of this kind it must be understood that 
great variations in the ash and ash content occur in 
the same species and even in different parts of the 
same tree. The non-mineral elements are about the 
same in all woods, being in the proportions : 



Carbon 49 per cent, by we 

Oxygen 6 per cent, by weight. 

Hydrogen 44 per cent, by weight. 

Ash, etc 1 per cent, by weight. 

The ash content may vary from 0.3 per cent, to over 
3 per cent. 



CHAPTER III 

Common Peactices in Drying 
Present practice in kiln drying varies enormously 
and there is no ''standard" method. Even with the 
same species and for the same purpose all kinds and 
conditions are met with. Temperatures vary anywhere 
from 60° F. to above the boiling point, and inch lumber, 
from one to eight months air seasoned, is dried in from 
thirty-six hours to six weeks, thicker material from 
two to five months. As a general thing, hardwoods are 
dried at a much lower temperature than softwoods, 
and for some softwoods, notably Douglas fir and south- 
ern yellow pine, temperatures above the boiling point 
of water are used. Such softwoods are usually dried 
directly from the saw in the perfectly green condition, 
as speed of drjdng is the main object aimed at, but 
with hardwoods, especially with the dense, heavy 
species, as oak, hickory, mahogany, etc., it is usual to 
allow them first to air dry by open piling in the yards 
or sheds from a month to one or two years before 
placing in the dry kiln. The reason for this will be 
explained in Chapter IV, ''How Wood Dries." Some 
of the softer "hardwoods" as basswood, poplar, yel- 
low poplar, etc., are also occasionally dried directly 
from the saw. It is possible, however, to obtain the 

23 



24 THE KILN DEYING OF LUMBER 

best results with all species by doing away with the 
air drying entirely and placing the wood in the kiln 
directly from the saw. This can only be accomplished, 
however, by using a proper method of drying best 
suited to the species, as will be shown later on. In 
air drying there is little or no control over the con- 
ditions of the air, which change according to the 
weather, so that the drying is likely to take place too 
rapidly on some days and too slowly on others. The 
result is liable to be surface and end checking, case- 
hardening and honeycombing (Fig. 8). In a properly 
operated dry kiln, in which the conditions are all under 
control, these injuries may be avoided. Some species, 
indeed, can not be successfully air dried under ordinary 
conditions, as they will honeycomb (internal checking) 
very badly. This is true of the southern oaks, espe- 
cially those white oaks containing large amounts of 
moisture, like the willow oak. It is also true of the 
blue gum or Eucalyptus glohulus. 

Another case which may be mentioned where air 
drying is injurious is black walnut gun stock blanks, 
which are cut into form while green and subsequently 
dried. This wood in this form will invariably end check 
in* air drying unless the ends are coated with an im- 
pervious covering. In fact, most of the hardwoods if 
cut into blanks while green, if more than an inch in 
thickness, will seriously check in air drying. This is 
true of maple last blocks, for instance. 



POORLY KILN DRIED 
SOUTHERN RED AND 
WHITE OAK WiGOH 

-ELLOES 



THE SAME. 
WELL KILN DRIED IN 
HUMIDITY REGULATED 

D5Y KILH 




^-^po\tleHltS^^^ honeyoomb:ng. The three speci- 

nght are kiln-riried oak wagon eUoes 3 - i^Phill^^' m section, the four specimens to the 
lower one white oak. ^^ '''•* *"'^^«=s m section, the top ones being red and the 



COMMON PRACTICES IN DRYING 25 

While green lumber can be better dried in the kiln 
if the fundamental requirements are all taken care of, 
preliminary air drying is unquestionably to be pre- 
ferred to bad kiln drying, or where there is any doubt 
in the matter. 

Preliminary Treatments to Drying. — Various pre- 
liminary treatments to kiln drying are sometimes used. 
Soaking in water is sometimes advocated, and has fre- 
quently been practised. In Japan and in Sweden it is 
customary to soak the wood in ponds or tanks for a 
year or longer before drying. Prolonged soaking 
leaches out soluble sugars, gums, and albumens from 
the sapwood, which may have a tendency to slightly 
reduce the hygroscopicity, but there would appear to 
be little if any benefit from the drying standpoint for 
most species, and the etfect is confined chiefly to the 
sapwood portion of the log.^ 

A research was carried on by Gabriel Janka as to 
the etfect upon the strength and the shrinkage of a 
number of species of wood produced by soaking them 
in soft and also in salt water. The experiment is 
described in full, together with tables of all the data in 
the reference given below.^ These investigators con- 
clude that the soaking in fresh water reduces the hy- 

* See also Chapter IV, " Soaking in Water." 

^ Mitteilungen aus dem Forstlichen Versuchswesen Osterreichs, Heft 
xxxiii, 1907. " Einwirkung von Siiss- und Salzwassern auf die Gewerb- 
lichen Eingenschaften der Hauptholzarten," by Gabriel Janka» 



26 THE KILN DRYING OF LUMBER 

groscopicity and the shrinkage and also lessens the 
checking to some extent. It deteriorates slightly in 
strength. 

When soaked in salt water it probably shrinks less 
than nnsoaked wood, but chiefly on account of an in- 
creased hygroscopicity produced by the presence of 
the salt in the wood. It swells more when exposed to 
dampness than unsoaked wood, but checks less. Soak- 
ing in fresh water is recommended by this author for 
wood used for technical and mechanical purposes. The 
table on pages 27 and 28 gives a brief summary of the 
results as to shrinkage. 

Some species of wood can be dried in the standing 
tree by girdling the trunk, cutting through the sap to 
the heartwood. This is the common practice in India 
with teakwood, where the trees are girdled and allowed 
to stand from two to three years before felling. The 
practice is impracticable in our present methods of 
lumbering, and would be detrimental in species where 
decay is liable to set in and borers are apt to attack 
the trees under the bark. In this connection it is in- 
teresting to note that the free water can be very quickly 
got rid of in the sapwood, if the trees are girdled or 
felled while in full leaf and allowed to remain until 
the leaves shrivel up before sawing into logs. 



COMMON PRACTICES IN DRYING 



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COMMON PRACTICES IN DRYING 29 

Preliminary treatments of steaming both below the 
boiling point and under pressures as high as 20 or 
even 40 pounds gauge are in use at the present time. 
The claim is made for the pressure treatment that the 
subsequent rate of drying is greatly increased thereby 
and shrinkage is reduced. The treatment is made in 
a tight cylinder where the wood is given the steam bath 
usually for twenty minutes at twenty pounds, but some- 
times for an hour or more, according to thickness. 
This heats the wood through to the center, and when it 
is brought out into the air the specific heat contained 
within the wood and water is capable of evaporating 
spontaneously a certain amount of moisture as the 
lumber cools to the atmospheric condition. If the wood 
is very "sappy," that is, if it contains considerably 
over 25 per cent, of its dry weight in water, the ex- 
pansion of the air in the cells mechanically forces out 
some of the free water or ' ' sap. ' ' Beyond this initial 
evaporation and expulsion of free water experiment 
does not show that any advantage is gained by this 
treatment in the drying. It does have a chemical effect 
upon the wood and darkens its color, particularly of 
the sapwood. This is of advantage in such woods as 
black walnut and red gum and mahogany. It reduces 
the strength and also the hygroscopicity of the wood, 
as will be shown in Chapter XI. 



30 THE KILN DRYING OF LUMBER 

CLASSIFICATION" OP METHODS OF DRYING AND TYPES OF KILNS 

By the term ''Dry Kiln" alone nothing more is 
conveyed than some kind of a room in which lumber 
may be placed and heat applied. A classification ac- 
cording to the shape and construction of the buildings 
or apparatus would be little better than a classifica- 
tion of ice boxes by their shape and color. There are 
at least twenty-five different makes on the market, and 
many of these embrace various kinds and sizes. A 
more logical way will be to classify them all according 
to the fundamental principles upon which they operate, 
or the process of drying. 

In the first place, we will consider those which are 
designed for drying lumber at the normal atmospheric 
pressure. These consist of a chamber or room, usually 
from 8 to 22 feet in width and 10 to 14 feet in height, 
of any required length, into which the lumber may be 
piled, or run in on trucks or bunks which run on rails. 
They are built of wood, brick, cement, plaster, hollow 
tile, or even sheet iron — in fact, of any construction 
material. There are two general forms in which they 
are built, depending upon the manner in which it is 
desired to handle the lumber, namely, in the form of a 
long chamber or hallway, usually from 60 to 150 feet 
long, having rails running the entire length, and doors 
at either end. The trucks of lumber are run into one 
end, moved along periodically, and taken out at the 



COMMON PRACTICES IN DRYING 31 

other end. The end at which the lumber is shoved in 
is known as the ''wet" or green end and the other as 
the ''dry" end. This is called a '^ progressive" form 
of kiln. The other kind consists of a suitable chamber, 
of the same cross sectional dimensions as the "pro- 
gressive" but usually much shorter in length, from 
18 to 70 feet, and with a door at one end only. This 
form is called a "charge" or "compartment" kiln. 
In it the lumber is placed and remains stationary until 
dry, the conditions of the air being changed accord- 
ingly, instead of moving the lumber along to succes- 
sively drier portions of the kiln as in the progressive 
form. The relative merits of the two forms will be 
discussed further on. 

Either form of kiln may be used with any of the 
processes. Preliminary steaming may be used with 
any one, and this is accomplished sometimes in a sep- 
arate steaming chamber, or cylinder, or in the kiln 
itself; if in a progressive kiln an end compartment is 
curtained off for the purpose, but if it is done in a 
compartment form the whole kiln is used for the 
steaming. 

A perusal of the records of the United States Patent 
Office shows that over a hundred different forms of 
dry kilns for drying lumber have been patented, as 
well as many of the accessories, such as doors, heating 
apparatus, trucks, loaders, etc. One of the earliest 



32 THE KILN DRYING OF LUMBER 

ones was taken out in 1862 and is operated primarily 
upon the principle of a hot-air furnace. In addition 
to the hundred or more stationary forms of kilns for 
drying lumber at atmospheric pressure, there are 
numerous patents for apparatus in which pressure and 
vacuum may be obtained and in which the apparatus 
is mechanically revolved or moved to secure thorough 
heating of the entire pile of wood or other hygroscopic 
material which is to be dried. Evidently, many of 
these have failed to produce the results anticipated or 
have been impractical from a commercial standpoint, 
since there are to-day only about twenty-five kinds in 
commercial use. 

Other Kinds of Driers. — While this publication will 
concern itself primarily with the subject of lumber 
driers, mention should be made of apparatus for dry- 
ing other materials. Where the wood is sufficiently 
thin, namely, quarter inch or less, it is usually dried 
in a totally different kind of apparatus, called a veneer 
drier. A typical form of this apparatus is shown in 
Figure 9. It consists of a long flue or box, about 50 to 
100 feet long by 8 feet high and 6 to 13 feet wide. 
Through this flue is carried horizontally on rollers a 
series of steel belts composed of short, narrow, flat 
links hinged together lattice fashion so as to allow of 
slight lateral motion, and to present a perfectly flat 
surface. The thin wood or veneer is fed into the 



|K^-¥^'ys''' 



J|^a^ M^iSSH 





Fig. 10. — Heated plate veneer drier. (Courtesy Merritt Mfg. Co.) 



COMMON PRACTICES IN DRYING 33 

machine at one end between two of these belts which 
press upon it and hold it perfectly flat as it is carried 
along through the flue by the motion of the belts. The 
lateral play of the steel links permits the veneer to 
shrink without imposing stresses upon it which would 
cause it to check. Heated air is forced through the 
flue across the veneer by a series of large fans and in 
a direction opposite to the motion of the belts, and high 
temperatures are used, frequently above the boiling 
point, since there is not the danger of injuring this 
thin material that there is in the case of lumber. The 
time of drying will depend upon the thickness and kind 
of material. Thin oak veneer may be run through this 
machine in twenty minutes and comes out flat and dry 
at the other end; one-thirtieth inch sliced mahogany 
may be dried in four minutes, one-twentieth inch in 
fourteen minutes, or one-eighth inch in about sixty 
minutes. 

Another form of veneer drier consists of a series 
of flat parallel iron plates, heated internally by steam, 
and arranged so as to open and close automatically 
and periodically like an accordion (see Fig. 10). 
The veneer to be dried is slipped in between these 
plates, which alternately close tight upon it, thus heat- 
ing it through, then open up and permit it to dry 
partially, when they again close and repeat the opera- 
tion until the veneer is dry. 



34 THE' KILN DRYING OF LUMBER 

Another kind of drier very similar to the lumber 
kiln is the varnish drier. This usually consists of a 
rectangular room, sometimes made simply of canvas 
stretched on wooden frames, of any suitable size, in 
which the freshly varnished furniture is placed, and 
a low heat maintained with a good circulation of air. 
It is important to retain a considerable amount of 
humidity so that the varnish will dry smoothly with- 
out crackling and to prevent the wood from shrink- 
ing. These varnish driers are usually placed inside 
of the shop or building, and are built dust proof. Some- 
times steam pipes for heating are placed on the sides 
and condensers on the ceiling, or the heating pipes may 
be placed beneath the floor and condensers on the sides, 
or ventilation may be used instead of the condensers. 
A temperature of 110° to 115° Fahrenheit and an 
initial humidity of 70 per cent, gradually reduced to 
30 per cent, are recommended for drying varnish on 
wood. Varnish on metals may be dried at higher tem- 
perature, 130°. A good circulation of air is required. 

Other forms of driers are used for drying loose, 
granular or pulverized material, as grain, hops, pow- 
ders, etc. These require mechanical stirring in order 
to break up the mass into which they settle and dry all 
portions. Some form of rotary drum is generally used. 

Evaporators, for evaporating solutions, are still 
another form of drier. Condensed milk is produced 



COMMON PRACTICES IN DRYING 35 

by evaporating the milk from shallow trays or pans 
in which a high vacuum is maintained so as to reduce 
the temperature of the boiling point. This subject has 
been extensively studied and books have been published 
which discuss it in full.^ 

Still other kinds of kilns requiring special con- 
struction are brick kilns and pottery kilns, but except 
for the fact that the underlying principles of their 
drying operations are somewhat analogous, it is hardly 
necessary to discuss them here. 

Classification of Lumber Drying Kilns. — Eeturn- 
ing to this subject of the drying of lumber, the kilns 
which operate at atmospheric pressure may be classified 
according to the method of operation as follows: 

1. Dry air or furnace (obsolete, except for small 

pieces). 

2. Moist air : 

(a) External circulation, partly returned: 

(1) Ventilated. 

(2) Forced draught or blower. 

(b) Internal circulation: 

(1) Condensing. 

(2) Humidity regulated water spray. 

(3) Humidity regulated steam spray. 

(c) Oven or boiling. 

' See " Evaporating, Condensing, and Cooling Apparatus," by E. 
Hausbrand, 1908, Scott Greenwood & Sons. 



36 THE KILN DRYING OF LUMBER 

3. Superheated steam: 

(a) High superheat forced draught. 

(b) Low superheat, high velocity internal circu- 

lation. 

The earliest kilns for drying lumber by the use of 
heat were probably developed in Europe and consisted 
of little else than a room in which a wood fire was built 
in such a way that the smoke and heated gases were 
made to pass through the material to be dried. The 
smoke had the added effect of darkening the wood and 
was supposed also to harden poplar and soft-textured 
woods. The earliest kilns used in this country were 
of a similar pattern, or instead of the gases of com- 
bustion being used directly furnaces were used to heat 
the air in the kiln, and fresh air was admitted from 
the outside.* 

These furnace or dry air kilns are now obsolete 
in this country, although they are still used to some 
extent in France. It is noteworthy that in the early 
days the idea of drying wood by heat was thought of 
mainly in connection with shipbuilding, where heat was 
already in use for bending the wood. 

The present idea of ''artificially" drying wood ex- 
tensively by means of dry kilns is of quite recent origin, 
dating back not much beyond the sixties. It is note- 
worthy, furthermore, that the United States is far 



* See " Timber," by Paul Charpentier, Scott Greenwood & Sons. 



COMMON PRACTICES IN DRYING 37 

aliead of all other countries in the development of this 
art. This is, no doubt, due to the economic conditions, 
the excessively rapid exploitation of the forests, the 
enormous consumption of wood per capita in the rapid 
settlement and development of the new country, 
amounting to seven times that of Germany and nine- 
teen times that of England, and to the necessity of 
reducing freight rates on shipments. 

Where wood is of high value and where no great 
rush was made in marketing the sawed lumber, great 
care was used in handling and in drying. Time was 
not so important a factor and the lumber could well 
be left in closed sheds to season gradually for two and 
even five or more years. Sometimes durable species, 
as oak and teak, were buried in earth or dung to cause 
them to season very slowly. But where such an 
enormous quantity of cheap building and construction 
material was called for on short notice, as in the rapid 
reconstruction period after the Civil War, and in the 
vast development of the unsettled western territories 
by the ' ' '49-ers, ' ' artificial drying by means of heat was 
the natural outcome, and having once seen the advan- 
tages of the process we have kept it up and improved 
it. The practice has indeed become an art and one 
which, nevertheless, when properly applied has truly 
many decided advantages over the old air seasoning 
methods. The contrast between the system used in a 



38 THE KILN DRYING OF LUMBER 

gigantic American lumber producing plant and the 
painstaking methods at a Grerman sawmill, where the 
boards from each log are carefully piled separately in 
the exact manner in which they were sawed from the 
log, is very striking. 

The modern development in kilns has been away 
from the dry air evaporation methods and towards the 
retaining of more or less moisture in the air or of 
adding moisture to the air before it comes in contact 
with the wood. The term ''moist air kiln" is com- 
monly used to convey this idea. Practically all of the 
recent patents are for some form of "moist air kiln," 
or for superheated steam methods. The moisture is 
usually obtained from the evaporation of the lumber 
itself, the air being entirely or partially recirculated 
instead of being all discharged outwardly. Sometimes 
moisture is added by allowing free steam to escape 
in the air or by bringing it in contact with a surface 
or a spray of hot water. 

These kilns may be divided into two groups, accord- 
ing to whether the air is recirculated entirely within 
the kiln itself, the kiln being closed to the outer atmos- 
phere, or whether the air is drawn out or allowed to 
escape from the kiln and fresh air is admitted. The 
latter class of kilns may be termed ''Ventilated'^ kilns, 
or ''blower" kilns, according to the system used in the 
air motion. The moist air is secured either by per- 



COMMON PRACTICES IN DRYING 39 

forated steam pipes in the drying chamber or in the 
inlet flue, or from the evaporation of the lumber itself, 
in which case a portion of the air is recirculated. In 
some makes the humidity is supplied by passing the 
air over a tray of water which may be heated more or 
less, according to the humidity desired, by means of a 
submerged steam pipe. 

Ventilated Kilns. — ^A well-known make of ventilated 
kiln of the progressive form is shown in Fig. 11. In 
this type fresh air enters at the dry end beneath the 
heating pipes through flues which extend about one- 
third the length of the chamber with openings on the 
upper side every little ways. The dry air, after rising 
through the heating pipes, passes through the piles 
of lumber in a horizontal direction towards the far end 
of the kiln. In taking up moisture from the lumber it 
becomes cooler and heavier and sinks towards the floor. 
It is then sucked out through openings near the floor 
into tall chimneys at this end. Dampers in these open- 
ings regulate the draught. The green lumber is shoved 
into the kiln on suitable trucks at the moist end and is 
moved gradually towards the dry end in a direction 
opposite to the current of air, whence it is removed 
when dry, in from several days to three or four weeks, 
as the case may be. It is thus seen that the entering lum- 
ber comes in contact with the dampest and coolest air 
first and is gradually moved into drier and hotter air. 



40 THE KILN DRYING OF LUMBER 

In another type of ventilated kiln the damp air is 
taken ont through a series of openings all along the 
side walls or in the ceiling. The air also enters through 
passageways in the walls, which discharge beneath 
the heating pipes arranged along the bottom. In some 
cases the moist air is sucked out from a series of flues 
from beneath the lumber and the fresh air is admitted 
near the top of the kiln. This latter method is the more 
logical, since, as will be shown later, the natural motion 
of the air in passing through the wet lumber is in a 
downward direction. The air motion through these 
flues is sometimes accelerated by placing heating pipes 
in their vertical extensions or by steam jets used as 
ejectors. 

It should be noted that in all of these ventilated 
kilns a natural recirculation of the air within the 
kiln itself, produced by the spontaneous heating and 
cooling by the steam pipes and by the lumber respec- 
tively, must be obtained in addition to the current of 
air entering and leaving the flues, as otherwise success- 
ful drying is impracticable. 

Still another form should be mentioned because it 
is quite distinct from the others just described, and its 
operation is very logical. In this kiln the damp air 
escapes through a narrow vertical passage on the side 
walls, but part of the air descends through a wider 
passage on either side to the bottom of the kiln from 



COMMON PRACTICES IN DRYING 41 

whence it enters beneatli the heating pipes and then 
rises, due to its being heated, passes through the 
lumber and again descends on the sides of the kiln; 
only the excess vapor produced by the evaporation 
escapes upwards and out under the eaves of the roof, 
since there is no air inlet in this kiln, except what occurs 
through leakages. It is suitable for drying wood which 
requires a very moist air. 

Numerous other forms of ventilated kilns exist, but 
it would be impracticable to describe them all here. 

Forced Draught Kilns. — In the forced draught kilns 
(Fig. 12), the heating apparatus is usually external 
to the kiln proper, being placed in the air duct, but 
not necessarily so. Heating pipes along the floor 
of the drying chamber may be used as in other 
forms. The main feature of this type consists in a 
forced draught qf air produced by a slight pressure by 
means of a fan or blower, instead of by the "natural" 
or gravity currents of air as in the other cases. Some- 
times the air is sucked out from the kiln, making the 
pressure within slightly less than atmospheric, and 
sometimes it is forced into the kiln, in which case the 
pressure within is greater than the outer air. In all 
cases it is necessary to properly distribute the air 
current by means of flues suitably arranged. In order 
to maintain humidity a portion of the air is usually 
recirculated ; by connecting the exhaust flue back again 



42 THE KILN DRYING OF LUMBER 

to the fan a damper is so arranged tliat more or less 
ontside air may be admitted to the fan as required and 
the moist air allowed to escape. In some oases instead 
of admitting fresh air to displace the moisture pro- 
duced from the evaporation, a closed circuit is used 
and the moisture is removed by means of condensers. 
The main trouble in all forced draught kilns appears 
to be in the difficulty of producing a uniform circula- 
tion through all portions of the lumber, without which 
uniform drying cannot take place. The reason for this 
is because the air motion is produced by diif erences in 
pressure between any two points in the kiln instead 
of by differences in density due to temperature, as in 
so-called ''natural draught" kilns. It is possible by 
a proper arrangement of flues and piles to combine the 
natural draught with the forced air movement and so 
obtain the advantages of both, but this has not usually 
been accomplished, due doubtless to lack of knowledge 
of the true factors influencing circulation. One type 
of blower kiln is shown in Fig. 12. 

Internal Circulation Kilns. — Internal circulation 
kilns are primarily of two kinds, namely, (1) with pipe 
condensers, (2) with water sprays. The latter is the 
kind developed by the author for the United States 
Forest Service, which will be more fully described in 
Chapter VIII. There is still another kind which has 
been used experimentally by the Forest Service, but 




^^^ 



Fig. 13. — Cross section of a ventilated type of kiln. (Courtesy of American Blower Co.) 



COMMON PRACTICES IN DRYING 43 

is not as yet in commercial practice. In this kiln direct 
sprays of live steam are used to force the circulation 
and they impinge directly upon water pipe condensers. 

In the internal circulation kilns the motion of the 
air is produced wholly or in part by gravity, due to 
differences in density brought about by heating and 
cooling effects. In the spray system this ** natural" 
circulation is greatly augmented by the force of the 
sprays. Where the pipe condensers alone are used the 
gravity effect must be wholly relied upon to produce 
the circulation. These kilns have the advantage over 
the ventilated type in that they are independent of 
external atmospheric conditions save for radiation 
through the roof and walls. The tighter they are 
closed the better. As a rule the compartment form 
of kiln is best suited for this method of drying, and 
is most frequently met with, although they are also 
used in the progressive form to some extent. 

A typical form of the condensing kiln has the heat- 
ing pipes located centrally beneath the lumber and 
the condensers on the side walls. In some kilns tlie 
condensers are placed in separate chambers on one or 
both sides, communicating with the drying chamber near 
the roof and beneath the heating coils; in others the 
condensers are on the side walls and shielded from 
the lumber by a thin partition of some kind, such as 
wood or asbestos board ; or again the condensers may 



44 THE KILN" DRYING OF LUMBER 

be wholly exposed. Some makes place the condenser 
near the ceiling, others near the floor. 

In some of the patented kilns, which do not now 
appear to be on the market, the condensers are placed 
beneath the lumber and the heating pipes on the sides. 
This is the most logical arrangement for green lumber, 
as will be seen in Chapter VI. The condensing kiln 
requires a considerable amount of cold water, for use 
in the condensers, since not only all of the moisture 
evaporated from the wood must be condensed and the 
latent heat absorbed by the water, but heat is also ex- 
tracted from the air as it passes downward over the 
condensers. Furthermore, the pipes being almost con- 
tinually wet are liable to rust very quickly. The 
humidity is controlled in condensing kilns by allowing 
more or less water to flow through the condensers. 
As in the case of the ventilated kilns, steam spray 
pipes are usually present for use where a high 
humidity is required. 

In the water spray humidity regulated kiln, invented 
by the author, the water sprays take the place of the 
condensers. The sprays are placed in a separate 
chamber or flue either on the sides of the kiln or in the 
center, and the force of the spray water greatly in- 
creases the velocity of the air. The humidity is very 
simply controlled by regulating the temperature of the 
spray water. The sprays saturate the air and deliver 



COMMON PRACTICES IN DRYING 45 

it beneath the heating pipes in a saturated condition at 
any required temperature. As the temperature of this 
saturated air is the deiu point of the air after it passes 
through the heating pipes, this may be termed the 
' ' dew-point method ' ' of controlling the humidity. The 
temperature of the spray water may be regulated auto- 
matically. As this kiln will be fully described in an- 
other chapter further description need not be given here. 
The steam spray internal circulation method is not 
on the market and is merely mentioned here as a pos- 
sibility, having been recently designed * and experi- 
mented with by the United States Forest Service. 
Steam is admitted through small perforations (about 
three thirty-seconds of an inch in diameter, spaced 
twelve inches apart) in a steam pipe in such a way 
that the force of the escaping steam shall add to the 
natural circulation of the air. In order to keep the 
humidity down to the required amount it is necessary 
to use pipe condensers, preferably placed directly in 
the spray, so that the steam impinges directly upon 
them. Evidently by this means all of the heat of the 
escaping steam and a little more has to be removed by 
the condensing water. A high internal circulation can 
be obtained by this means with about twenty pounds 
gauge steam pressure and the humidity can be uni- 
formly maintained by adjusting the condensers. Ex- 
cellent drying results have been obtained, but the 

* By J. E. Imrie. 



46 THE KILN DRYING OF LUMBER 

method seems wasteful of steam and condensing water, 
unless the large volume of hot water can be made use of. 

Boiling, or Superheated Steam Kilns. — For wood 
which will stand the high temperature a very quick way 
of getting rid of the free water is to heat the lumber 
above the boiling point. A method used somewhat 
extensively on the northwest coast for drying green 
Douglas fir and other softwoods to the shipping weight 
is to close the kiln fairly tight, allowing only for the 
escape of vapor formed by evaporation, and to allow 
the temperature to rise above the boiling point. The 
internal circulation or convection appears to be suffi- 
cient to heat the lumber, but unless the pile is very 
open, the inside does not dry uniformly. Inch lumber 
is reduced from 32 per cent, to 10 per cent, of the dry 
weight in 40 to 43 hours. Sometimes live steam is used 
at the start of this process to saturate the air. 

The ordinary ventilated type of progressive kiln 
with steam pipes in the bottom is used for this method 
by closing all the ventilators. Time of drying varies 
in these *'oven" or "boiling" kilns from twenty-two 
hours to five days for inch lumber. Edge piling or 
very open flat piling gives the most uniform results. 

Another application of the boiling principle is in 
the direct use of superheated steam. This is gener- 
ally accomplished by forcing the superheated steam 
into the kiln by means of a blower. A form commonly 
used is one in which the steam at atmospheric pressure 



' COMMON PRACTICES IN DRYING 47 

is superheated by passing it around a brick oven and 
thence into the kiln. Since the vapor pressure of 
water at 212° F. is equal to the atmosphere, it follows 
that when the vapor is heated to 212° or above, the air 
becomes entirely displaced and drying occurs in vapor 
alone. The only drying capacity which the vapor has 
is the heat which it contains above 212°, since at that 
temperature it becomes saturated and no drying can 
take place. It is therefore necessary to use a high 
degree of temperature or else an enormous circula- 
tion to accomplish the drying of the lumber. This will 
be more fully discussed in Chapter IX. 

Drying in Superheated Steam. — ^A temperature of 
300° F. is in use by this method. It is the most rapid 
method of drying there is, but since the steam loses its 
superheat with exceeding rapidity, the greatest effect 
is upon that portion of the lumber only with which it 
first comes in contact, since its drying capacity entirely 
disappears as it cools to 212°. For this reason uneven 
drying is likely to result. Thus, in the case of one-inch 
red fir, which was dried from an average moisture 
content of 34 per cent, to an average of 7.5 per cent, in 
twenty-three hours, a variation from 2 to 18 per cent, 
occurred in boards sampled from different portions of 
the pile, with flat piling. As the lumber becomes dry, 
its temperature will rise to that of the steam, so there 
is danger of injury to the wood. 

High Velocity Low Superheat Method. — An im- 



48 THE' KILN DRYING OF LUMBER 

provement on the blower method has been developed 
by the author and his co-workers, Mr. Norman Betts * 
and Mr. James Imrie, for the Forest Service, in which 
a high velocity of superheated vapor is produced within 
the kiln itself by means of steam jets and heating pipes 
suitably arranged. Since the drying capacity of water 
vapor is directly proportional to the product of the 
number of degrees of superheat above 212° F. by the 
quantity circulated, it has been found possible to 
greatly reduce the temperature by thus increasing the 
velocity, without decreasing the drying eifect. While 
this process has not yet been put into commercial opera- 
tion, it has been proved applicable on a commercial scale 
through experiments recently concluded in the South on 
southern pines= Grreen inch lumber has been dried in 
full-sized kilns to shipping weight in 24 hours by this 
method. The sapwood of southern pines is darkened, 
otherwise the lumber comes out in perfect condition. 
A recent improvement in this method,^ for use with flat 
piling, is shown in Fig. 14, in which the steam jets are so 
placed that the circulation is readily reversed in direc- 
tion. It has been demonstrated to be successful on a 
commercial scale. It should prove particularly useful 
for some of the West Coast lumber, as Douglas fir, 

* With deep sorrow, his death by lightning, since the above was 
written, is announced. Mr. Betts was a young man of marked ability 
and an unusually pleasing personality. 

^ Patent applied for, Serial No. 149,972, Feb. 20, 1917. 



COMMON PRACTICES IN DRYING 



49 



white fir, red fir, Sitka spruce, Alaska cedar, and other 
firs and spruces, and incense cedar. 

In experiments made at the Forest Products Lab- 
oratory in Madison, Wis., incense cedar was dried from 
a weight of 6079 pounds per ^ feet to 1982 pounds in 




^:i^\/V.V^ ' /:'.<:'.-^ !.-rw 

l>-v'.^~~ "-^ — *-~ — \<t-yA 

Fia. 14. — Flat piling. 

48 hours, or an average rate of 83 pounds per ^ board 
feet per hour, by far the most rapid drying of which 
there is any authentic record for a pile of lumber of a 
semi-commercial size. The following species of wood 
have been successfully dried by this method. The time 
of drying, reduction in weight, and percentage of de- 
grade are also given in Table IV. 



50 



THE KILN DRYING OF LUMBER 



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COMMON PRACTICES IN DRYING 51 

It should be noted that superheated steam methods 
are not suitable to the majority of hardwoods nor to 
many of the softwoods, and are suitable only for dry- 
ing green lumber to shipping condition where the 
quickest possible method is called for. For high quality 
woods the darkening effect and the weakening of the 
fiber, which generally result even where there is no 
injury in checking, or loosening of knots which often 
takes place, are apt to be objectionable features. 

Estimate of Saving hy Kiln Drying. — The follow- 
ing example is given in order to illustrate how the kiln 
drying of wood may prove financially profitable, aside 
from the question of the necessity of having kiln dry 
wood or of the saving in losses such as checking, rotting, 
staining, warping, etc., which can be accomplished by 
proper methods of kiln drying. 

The exact figures will, of course, vary greatly with 
local conditions, so that in giving this example it must 
be understood that widely different resnlts will be 
found for each individual case, but the various items 
can be adjusted accordingly to fit the local conditions. 

Assume a plant built for continuous operation for 
an indefinite time and handling on the average of 
100,000 board feet per day of hardwood lumber, such 
as oak, chestnut, gum, etc., or 250 days in the year 
equivalent to 25,000,000 feet per year. 

For air drying one-inch lumber to shipping weight 
will require approximately nine months, to a moisture 



52 THE KILN DRYING OF LUMBER 

condition of 18 per cent, of the dry weight. To thor- 
oughly air dry would require from twelve to eighteen 
months. This material can be kiln dried to 6 per cent, 
moisture from the saw as an average in approximately 
three weeks. Saving of time at least eight months. To 
keep eight months' supply on hand requires a yard 
capacity for storing 240 X 100,000 or 24,000,000 board 
feet. To kiln dry 100,000 feet per day will require 
twenty trucks each of 5000 feet capacity per day. This 
will require a kiln capacity of 21 X 20 = 420 trucks, 
each load being about 8 X 8 X 16 feet in size. Com- 
partment kilns end piling 12 feet wide, 12 feet high, 64 
feet long, would hold four trucks each, requiring 105 
kilns. Progressive kiln cross piling twenty-one trucks 
per kiln 20 feet wide, 12 feet high, and 168 feet long, 
requiring twenty kilns. The latter, if built of masonry 
with equipment, would cost about $140,000, the former 
about $210,000. 

AiB Drying Costs (Annual) Saved by Kiln Drying. 
Capital tied up in 24,000,000 feet of lumber in yard at $20 

per M $480,000 

Interest at 6 per cent 28,800 

Insurance and taxes at $1.50 per $100 per year 7,200 

Yard expenses (from chains to cars, including sorting and grad- 
ing, piling and unpiling), $1.64 per M° 41,000 

Shipping weights saved (18 — 6=: 12), 12 per cent, of the dry 
weight of the lumber. Suppose average dry weight to be 
2600 pounds per M, the saving is 312 pounds per M. At an 
average of 25 cents per 100 pounds = 78 cents per M or 
25,000 X 78 per year 19,500 

* From actual figures in oak and gum in Arkansas. 



COMMON PRACTICES IN DRYING 53 

To carry 24 million feet in yard requires about 2400 piles, or a 
space of 13 acres. Skids for piles, tracks, hydrants, and 
general upkeep will amount, without counting rental, to an 
annual charge of $2,000 

Premium on lumber due to kiln drying, an average of $2 per 

M on 25,000 M 50,000 



Annual saving by kilns over yard drying $148,500 

Costs of I^lns. 

Investment in kilns $140,000 

Storage sheds to hold lumber 10 days, 1,000,000 feet. . 8,000 
Trackage and extras 2,000 

$150,000 
Extra boiler capacity 320 lb at $13.50 per ib = $4,320. 

Say with equipment 5,000 

$155,000 
Interest at 6 per cent. \ 

Insurance and taxes at 6 per cent. L 19% per cent, on $155,000 . $30,225 
Depreciation 7'^ per cent. \ 

Handling lumber: " 

Chain to kiln truck $0.12 

Loading in kiln truck 40 

Unload and pile 50 

Pile to R. R. car 50 

Sorting and grading , .15 

$1.67 
25,000 M per year at $1.67 41,750 

$71,975 
Cost of operation of kilns, attendance, water, etc. Estimate. . 12,500 

Total annual costs $84,475 

Total net annual saving due to kilns, $148,500 less 

$84,475 $64,025 

Or a net annual saving of $2.56 per M. 

^ From actual figures. 



54 THE' KILN DRYING OF LUMBER 

Of course, if the operation is only a brief one, for 
three or four years, the total investment in the dry kilns 
would have to be depreciated in that time or down to 
its ultimate sales value, which would probably throw 
the balance over on the other side. It is unsafe to make 
any general statement as to the financial benefits to be 
derived from kiln drying green lumber, but it is very 
often imperative to kiln dry the air-dried lumber any- 
way for the purposes required, and in that case, where 
some kind of a dry kiln plant is necessary in addition 
to the air drying yard, there is pretty certain to be a 
considerable margin on the side of kiln drying directly 
from the saw sufficient to cover all costs of more ex- 
pensive equipment for the green drying. 

Losses in Air Drying. — The above calculations did 
not take into consideration the saving in the losses 
which commonly occur in air drying, which may be 
obtained by proper kiln drying methods. These losses 
are much greater than commonly supposed, for the 
reason that there is seldom kept a record of them. 
They vary greatly with different woods and at differ- 
ent places. They are greatest in hardwoods, as a rule. 
An estimate of a total loss of 10 per cent, in air drying 
on all hardwoods is probably well within safe limits, 
and in softwoods from 4 to 7 per cent. In some in- 
stances losses have been more carefully estimated as 
follows : 



COMMON PRACTICES IN DRYING 55 

Red gum 1^ to 30 per cent. loss. 

Western larch (upper grade) 60 per cent, degrade. 

Southern pine 4 per cent. loss. 

Southern oak from 10 to 30 per cent. lo&s. 

Western white pine from 10 to 30 per cent. loss. 

Shaped hardwood blanks from 10 to 60 per cent. loss. 

Applewood for instruments 60' per cent. loss. 

While it is not probable that this loss can be entirely 
overcome by kiln drying directly from the saw, experi- 
ence and experiment have shown that it can be reduced 
to average less than 4 per cent., and in many cases less 
than 2 per cent. As there are about 4,725,000,000 board 
feet of hardwoods kiln dried per year, and 15,116,000,000 
feet of softwoods, it might be possible to save 10 per 
cent, on the value of hardwoods and about 3 per cent, 
on the softwoods. Taking an average price of $21 
for the former and $16 for the latter, this would amount 
to a possible annual saving of $2.10 per M for hard- 
woods and $0.84 per M on softwoods, or a total for 
the United States of $17,178,000, if dried directly from 
the green condition by the best dry kiln methods. 

This alone, without considering the question of sav- 
ing of freight rates and investment in outstanding stock 
calculated above, would seem to justify the use of cor- 
rect dry kiln methods and the employment of technical 
drying experts to look after this part of the lumber 
manufacturing business, instead of leaving the drying 
operation to take care of itself or leaving it to the 
engine tender or the yard foreman, as is commonly done. 



56 THE KILN DRYING OF LUMBER 

HEATING IN DEY KILNS 

The most common means of heating the lumber is 
by steam pipes running lengthwise of the kiln near 
the floor. One-inch or inch-and-a-quarter pipe has 
proved the most economical for the purpose. The 
pipes are usually built up in two or more sections, with 
separate headers and valves for each. A common 
arrangement is for the pipes to start from a supply 
header at one end, and bend down through a right angle 
elbow at the other end into a drainage header, from 
which a drain leads to a steam trap or a pump. The 
bend of the pipe is necessary so as to allow for ex- 
pansion of the individual pipes, and the vertical arm 
should be at least twelve inches long. An excellent 
joint at the bend in place of the plain elbow is to run 
a street elbow into a plain elbow, thus making a swivel- 
joint and avoiding all strains at the bend and danger of 
cracking the elbow. 

Sometimes continuous coils of pipe are used with 
return bends, made up in sections, in place of the 
headers and multiple pipes. This is perhaps the most 
desirable form for flexibility and uniform heat distribu- 
tion, but is a little more trouble to make. If made up 
in continuous coil form, the coils must be properly 
arranged for drainage, which is important in all heating 
installations. 
. If a steam trap is used instead of an exhaust pump, 



COMMON PRACTICES IN DRYING 57 

for the heating system, it is essential to have air valves 
placed at all high points, wherever there is any danger 
of air trapping, and the trap should always be by- 
passed so that the coils may be blown out occasionally. 
It is exceedingly important with the header system that 
the traps be working properly at all times, otherwise 
parts of the system may be cold while the rest is hot. 

It generally pays to use the best quality wrought 
iron pipe (not the trade name ''wrought pipe," but 
wrought iron pipe), as the piping in a dry kiln is sub- 
jected to excessive rust-producing conditions, and re- 
pairs are not as a rule easy or convenient to make. The 
reliable dry kiln manufacturers supply an excellent 
grade of pipe. 

A caution here is made against the use of galvan- 
ized iron pipe either for heating or condensing, as the 
zinc coating will not hold up in a moist air dry kiln, 
probably due to some action of the acid fumes from the 
wood. The zinc very quickly turns to a dry powder 
and rubs off. Plain uncoated iron lasts better than 
galvanized iron. The best covering for the pipes, if 
one is used, is a high melting asphaltum or a black 
baking japan. 

Often two horizontal layers of pipes are used, some- 
times three or four layers and often only one. In 
any event, the pipes should be arranged so that they 
are all accessible for repairs. In progressive kilns the 



58 THE KILN DRYING OF LUMBER 

pipes usually start from tlie dry end and run only 
about two-thirds or four-fifths the length of the kiln. 
Many different arrangements of pipe are in use and 
a great many patents have been taken out for the 
piping of dry kilns. In one arrangement, the pipes are 
placed in vertical tiers, thus making the individual 
units readily accessible, but at the same time losing a 
little in heat efficiency. After all, it does not much 
matter what plan of piping is used, provided it be 
arranged in a convenient and systematic manner. 

The quantity of pipe can not be predicted in ad- 
vance, since it will vary greatly in different cases and 
for different classes of kiln construction and climate, 
as well as the arrangement of the pipes. 

For low-pressure exhaust steam at least double the 
amount of piping is necessary for medium temperature 
than if boiler pressure is used in the kiln, for the radia- 
tion is nearly proportional to the difference in tempera- 
ture between the steam and the surrounding air. For 
slow, low temperature drying, as for green oak at 110°, 
very few pipes are necessary, not more than sixteen 
for a kiln twenty feet wide by twelve feet high above 
the rails. For high temperature drying at 180° or 
above the boiling point high-pressure steam is essential. 
A general rule to go by for drying at 130° to 140° with 
steam at seven pounds pressure, or at 180° to 220° 
with boiler pressure, is to use one piece of one-inch pipe 



COMMON PRACTICES IN DRYING 59 

for every two square feet cross section of kiln taken 
above the piping ; in other words, one foot of inch pipe 
for every two cubic feet of space. Thus, for a kiln 
twenty feet wide and thirteen feet high above the pipes 
130 one-inch pipes would be about correct. Since 2.9 
lineal feet of inch pipe is equivalent to one square foot 
external radiating surface, the above is equivalent to 
a heating surface of 0.1725 square foot per cubic foot 
of space above the pipes. It is always safe to have 
ample heating surface made up in several units, as it 
is easier to turn off some of the heat, if necessary, than 
to add it if deficient. 

Examples of Heating Capacities. — In certain dry 
kilns the following amount of heating surface gave the 
temperatures indicated : 

A. Sixty-four concrete compartment condensing 
kilns in one unit 18 feet wide, 15 feet above pipes, 30 
feet long, 18-inch condensing chambers on either side, 
38 one-inch pipes in each kiln. Exhaust steam used 
two to three pounds gauge. Temperatures in kilns 
80° to 110° Fahrenheit. Located in Indiana. 

Lengths of inch pipe to 1 cubic foot space above pipes between 

partitions of condensers 0.141 

Square feet of heating surface to 1 cubic foot sp,ace 0.0485 

B. Two Standard hollow tile kilns, tile roof over- 
laid with three-inch cinder concrete slab and four-ply 
waterproofing 18 feet wide, 13 feet above pipes and 150 



60 THE KILN DRYING OF LUMBER 

feet long. 15,150 lineal feet of inch pipe in each. Ex- 
haust steam at three pounds gauge used. Tempera- 
ture at dry end 160° F. Located in Ohio. 

Length of inch pipe to cubic foot space 0.429 

Square feet heating surface to cubic foot space 0.149 

C. Brick compartment, condensing, with partitions 
on either side 17l^ feet between partitions, 13 feet 
above top layer of pipe 50 feet long. Steam at 7^ 
pounds gauge. Temperature 150° and 53 per cent, 
humidity ; in Virginia, outside temperature being about 
30° Fahrenheit. 

Linear feet pipe per cubic foot space 0.361 

Square feet heating surface per cubic foot space 0.124 

D. Brick and concrete condensing chambers on 
either side, 20 feet wide between partitions, 14 feet 
above pipes 104 feet long; in northern Wisconsin. 
(Winter usually 20° below zero.) 

At greeti end steam 5% pounds gauge gave 150°, 95 per cent, 
humidity : 

Linear feet pipe per cubic foot space 0.28 

Square feet heating surface per cubic foot space 0.098 

At dry end, 5% pounds gauge, 165°, 60 per cent, humidity: 

Linear feet pipe per cubic foot space 0.64 

Square feet heating surface per cubic foot space 0.222' 

E. Four hollow tile, condensing both sides ; no side 
partitions, 20 feet Avide, 11 feet above rails, 124 feet 
long. Coils throttled and vacuum pump used. When 
full of green lumber 110° at 70 per cent, humidity. 



COMMON PRACTICES IN DRYING 61 

130° and 30 per cent, humidity at dry end. In Massa- 
chusetts. (0° F. in winter.) 

Lineal feet inch pipe per cubic foot space 0.585 

Square foot heating surface per cubic foot space 0.202 

An excellent way to regulate the heat is by means 
of a reducing valve on the main steam line by which 
any desired temperature may readily be produced by 
changing the steam pressure accordingly. The best 
results are obtained by using two reducing valves in 
tandem, the first one reducing into a capacity, such as 
a large piece of pipe, and the second reducing further 
to the required pressure. By thus stepping down in 
two stages from the boiler to the required pressure 
perfectly uniform pressure may be maintained. 
Another good method is to use a thermostat on the 
main steam line ; but if a thermostat is used it should 
be of the very best make, for should it fail to operate 
at any time there is danger of overheating the kiln. 

Calculations for Heating Capacity. — The following 
example will show how the heating capacity for a dry 
kiln may be calculated, although, as stated, there are 
so many variables that the result is after all little better 
than a guess, yet it will serve to indicate something as 
to possibilities. 

Let us assume that 50,000 feet of one-inch yellow 
pine are to be dried, in four days, or at the rate of 
12,500 feet per day, from an average moisture content 



62 THE KILN DRYING OF LUMBER 

of 70 per cent, to shipping weight of 20 per cent., and 
at a temperature of the entering air of 212° Fahren- 
heit. Let us suppose that the air enters the steam 
coils in a saturated condition at 70° and leaves the 
kiln three-quarters saturated, 75 per cent, relative 
humidity. The 50,000 board feet will represent, if the 
lumber is fifteen-sixteenth inch thick, about 3900 cubic 
feet. Dry yellow pine weighs from 38 to 42 pounds per 
cubic foot, say 40 pounds, and since there is 50 per 
cent. (70—20 per cent.) water to be extracted the weight 
of water is 20 pounds per cubic foot, or a total weight 
of 3900 X 20 = 78,000 pounds of water to be evaporated 
in four days. 

In considering the heat quantities involved, the 
radiation from the kiln walls will not be included in 
the consumption, because the heat remaining in the air 
leaving the lumber will supply this loss and need not 
add to the quantity consumed, since if it is not utilized 
in heating-the kiln walls it must be lost into the outside 
air or consumed by the condenser. Nevertheless, there 
will, no doubt, be some loss of direct heat through the 
walls. Now to evaporate one pound of water at 212° 
would require one pound of steam, if it could all be 
utilized for this purpose, or 966 British thermal units. 
But this is not the case when air is present, since some 
heat is utilized in heating the air. The theoretical 
amount has been worked out in Chapter X and is given 



COMMON PRACTICES IN DRYING 63 

in Table IX. Taking the nearest value to our given 
conditions from this table, we find that saturated air 
entering at 59° F. heated to 212°, and leaving 75 per 
cent, saturated has a leaving temperature of 103°, and 
that 1572 heat units are required to evaporate one 
pound. This value includes the heat consumed in rais- 
ing the temperature of the water from 59° to 103°, 
which must be included. Therefore, to evaporate the 
78,000 pounds will require a minimum of 122,616,000 
B.t.u. in 96 hours, or 1,277,250 B.t.u. per hour. This 
represents 1277 pounds of steam consumption per hour 
(total heat of steam above 212° at 307° = 1000) or 
1317 pounds at 212°. Taking a boiler horsepower as 
34.5 pounds evaporated from and at 212°, it repre- 
sents 38.3 boiler horsepower. This is the minimum 
possible amount, and it is probable that in most 
icases at least 50 per cent, more than this must be 
figured upon, or 1915.5 pounds steam per hour, or 
a total of 183,885 pounds of steam, or 57.3 boiler 
horsepower. 

For a longer drying period than four days, or for 
less moisture in the green lumber, the steam consump- 
tion per day will be proportionately less. 

For the amount of heating pipe required, several 
factors come in to greatly modify this, steam pressure, 
velocity of air passing over the pipe, and the tempera- 
ture of this air; that is, whether the entire amount is 



64 THE KILN DEYING OF LUMBER 

fresh cold air or whether some of the heated air in the 
kiln is recirculated; also the humidity of this air. In 
order to carry a temperature in the kiln of 212°, boiler 
pressure steam is necessary, because exhaust steam is 
net hot enough. The heat radiated from iron pipe 
varies for different sized diameters up to 12 inches. 
Nystrom ^ gives the following formula : 

Heat radiated per square foot of heating surface per 
hour in British thermal units = 

.001122 { 450+ (12-D)2 j X (T— 1)° 

when 

D = outside diameter of pipe. 

T = temperature of steam in F. degrees. 

t = temperature of air passing over pipes. 

n = a numerical exponent depending upon velocity of air. 

The following are the values given for n : 

Calm n =1 1.20 

Gentle n =: 1.22 

Brisk n = 1.24 

High n=: 1.26 

Assume that one-inch pipe is to be used, the area in 
square feet per 100 feet length is 34.5 and D is 1.315 
inches. The heat radiated per 100 feet length is then 

34.5 X .001122 5 450+ (12-1.315)* | (T-t)n = 21.84 (T-t)n 

For various sized pipes the coefficient for multiplying 
(T-t)*^ per 100 foot length is : 

•Nystrom Pocket Book. 



COMMON PRACTICES IN DRYING 65 

% 14-3 

% 17.62 

1 21.84 

11^ 27.36 

1% 30.80 

2 37.87 

2^ 45.06 

3 55.71 

For an ordinary kiln take n as 1.22. Suppose the 
effecting steam pressure to be 60 pounds = 307° F. = T. 
The entering air is 70° and it is to be heated to 212° 
before it leaves the steam coils, so that the air coming 
in contact with the pipes will be saturated and at a 
temperature of 70°. The equation then becomes 

11 = 21.84 (307-70)^'^=: 17,236 

heat units per hour per 100 feet of one-inch pipe im- 
parted to the air, (This is equivalent to a radiating 
factor of 2.11 heat units per square foot per degree 
difference in temperature.) 

The amount of heat required was found to be 1,277,- 
250 X 1.5 = 1,915,875 B.t.u. per hour, which divided 
by 17,236 gives 111.15 hundred-foot lengths of pipe, or 
11,115 feet of one-inch pipe. 

Let L equal the total heat above 212° contained in a 
pound of steam at temperature T = 1000 B.t.u., and 
W = weight of steam condensed per hour. Then 

N 17,236 
W = J = -j^ = 17,236 pounds 

steam condensed per hour per 100 feet length of one- 
inch pipe, or a total of 111.15 X 17.236 = 1915.8 pounds 
per hour, or 31.93 per minute. 
5 



66 



THE KILN DRYING OF LUMBER 



To find the size of steam main desirable to supply 
31.93 pounds of steam per minute, we must assume 
what drop in pressure is permissible for the given 
length of feed pipe. This may be calculated from 
Babcock's formula: 



w= 




D(Pi-Pi.)d« 
LC + f) 



Where W is the weight of steam in pounds per minute 

Pi is its initial pressure and Pg its final pressure after 

passing through the length of pipe L in feet. D is the 

weight of a cubic foot of steam at the initial pressure 

Pj and d is the diameter of the pipe in inches. To 

simplify the calculation the following table has been 

worked out from this formula for a length of pipe, L, 

of 100 feet and a difference of pressure P1-P2 of one 

pound : 

Table V. — Flow of Steam in Pounds per Minute Through a Straight 
Pipe 100 Feet in Length with a Reduction of Pressure of 1 
Pound per Square Inch. 









Nominal diameter pipe 




















pressure 


1 


IK 


1 1>^ 1 2 


2>^ 1 3 


4 


6 


8 


Pounds 

per 
square 

inch 


Actual diameter, inches 


1.05 


1.38 


1 1.61 


2.07 


2.47 


3.07 


4.03 


6.06 


7.98 





.90 


1.98 


3.06 


6.26 


10.27 


18.83 


39.82 


120.3 


253 


10 


1.15 


2.52 


3.89 


7.96 


13.06 


23.94 


50.62 


153.0 


321 


20 


1.35 


2.96 


4.58 


9.36 


15.37 


28.18 


59.58 


180.1 


378 


30 


1.51 


3.31 


5.12 


10.48 


17.20 


31.52 


66.65 


201.0 


423 


40 


1.67 


3.67 


5.67 


11.59 


19.03 


34.88 


73.76 


222.4 


468 


50 


1.81 


3.97 


6.14 


12.47 


20.59 


37.75 


79.82 


240.7 


506 


60 


1.93 


4.24 


6.56 


13.42 


22.03 


40.38 


85.39 


258.0 


542 


70 


2.04 


4.48 


6.94 


14.18 


23.28 


42.68 


90.24 


272.7 


573 


80 


2.16 


4.74 


7.33 


14.98 


24.59 


45.08 


95.32 


288.1 


605 


90 


2.26 


4.96 


7.67 


15.69 


25.75 


47.20 


99.80 


301.6 


633 


100 


2.37 


5.20 


8.04 


16.45 


26.99 


49.48 


104.60 


316.2 


664 



COMMON PRACTICES IN DRYING 67 

For lengths of pipe other than 100 feet, divide 
the values given in the table by one-tenth of the square 
root of the given length. Thus, for a pipe 144 feet long 
divide by 1.2. 

For any other difference in pressure, multiply by 
the square root of this difference. 

The resistance due to a globe value and to the 
entrance of the pipe may be taken each as equal to a 

9.5d 

length of pipe in feet of ., , 3.6\ and an elbow as two- 

thirds of a globe valve. The added length equivalents 
in feet for a globe valve according to this are : 

Nominal diameter of 

pipe 1 11/4 1% 2 21/2 3 4 6 8 

Equivalent length . . . 2.35 3.6 4.7 8.7' 9.6 13.4 20.2 36.0 52.0 

Thus for a four-inch pipe 100 feet long having one 
globe valve and three elbows, the resistance is equiva- 
lent to 4 X 20.2, or an added length of 80.8 feet, or is 
equivalent to a straight piece of pipe 181 feet long. 
Thus the actual drop in pressure as compared to the 
unbroken pipe 100 feet long would be 1.81 pounds per 
square inch, for the amount of flow given in the table, 

or it would deliver a flow of } = .743 times the 

1/1.81 

values given in the table for the 100 feet unbroken pipe. 

According to the calculation on page 66, 1915 

pounds per hour of steam is required, or 31.9 pounds 

per minute. From Table V it is seen that at 60 pounds 



68 THE KILN DRYING OF LUMBER 

pressure a three-inch pipe 100 feet long having one 
globe valve and two elbows would deliver .743 X 40.38, 
or about 30 pounds with a drop of one pound in pres- 
sure. A three-inch pipe would, therefore, be ample 
if the distance is not long, but if the length is greater 
than 100 feet or there are more resistances, a four-inch 
pipe would be better. 

Approximate Calculation. — The calculation may be 
more roughly approximated by assuming a radiation 
of 2.1 heat units per hour per square foot heating sur- 
face per degree difference in temperature. Let X = 
heating surface required, for a difference (T-t) = 237°, 
then 2.1 X X X 237 = 1,915,875. X = 3849 square 
feet. 2.9 lineal feet of one-inch pipe =; 1 square foot 
heating surface. Therefore, 4491 X 2.9 = 11,162 
lineal feet pipe required. This calculation all depends 
upon choosing the proper radiation coefficient 2.1, which 
may vary from 1.5 up to 3.0 according to the size, 
arrangement of piping, and the velocity of the air. 

For low pressure steam heating the diameter of 
the supply pipe in inches is often taken as one-tenth 
the square root of the heating surface in square feet 
1/10 Vss^= 6.2 inches. 

Kiln Capacity. — The size of the kiln would depend 
somewhat upon its construction and method used in 
drying, and whether the lumber is in even lengths. Let 
us assume that the lumber will be flat piled with one- 



COMMON PRACTICES IN DRYING 69 

inch stickers between the boards and that a ventilating 
type of kibi is to be used with cross piling, the lumber 
to be 16 feet in length. To obtain satisfactory results 
by this method the piles must not be too high and a 
space must be left between each truck and the boards 
spaced two or three inches apart. To allow clearance 
the kiln will need to be at least eighteen feet wide and 
at least two feet between the top of the pile and the 
roof. A space of about two feet will also be required 
below the rails and about a foot for the rails, trucks, 
etc. Thus the kiln must be twelve feet high for seven- 
foot piles of lumber. Suppose each truck is six feet in 
width, it will then hold approximately a pile of lumber 
7 X 6 X 16 feet, which if the boards were close-piled 
with one-inch stickers would contain i^ X 7 X 6 X 16, 
or 336 solid cubic feet. Allowing about 1,4 of this for 
the spaces between boards, each truck would hold 252 
cubic feet oif 3024 board feet. It would, therefore, 
require sixteen trucks to hold the 50,000 feet. Allow- 
ing a space of one foot between each truck and the 
ends of the kiln the length required would be 113 feet. 
This kiln would be operated progressively at the rate 
of four trucks per day. Or four compartment kilns 
twenty-nine feet in length and thirteen feet high by 
eighteen feet wide, each holding four trucks, would 
serve the same purpose. 

Another Example. — The above example was given 



70 THE KILN DRYING OF LUMBER 

for rapid drying at a high temperature. Let us take 
another case where the entering temperature is to be 
158° F. and the humidity 63 per cent., the air being 
recirculated and entering the heating pipes in a satu- 
rated condition at 140°. Our choice will lie between 
some kind of a condensing kiln or a recirculating blower 
kiln. Let us suppose that green inch birch is to be 
dried from 60 per cent, moisture to 30 per cent, in six 
days, or at an average rate of 5 per cent, per day, and 
then further dried to 6 per cent, at a higher tempera- 
ture, which will require a considerably longer time. 
Let us also assume 50,000 feet of inch lumber are to 
be handled in this length of time. Assume that the 
temperature of the lumber when brought into the kiln 
is 0° F. and the lumber is solidly frozen. For every 
pound of dry lumber, therefore, there will be 0.6 pound 
of ice to be heated from 0° to 32° and melted at 32°, 
then the lumber and water must be heated to 140° before 
any drying begins. Half of this water is then to be 
evaporated and this feat accomplished in six days. The 
heat necessary to heat the lumber and its contained 
water to 140° per pound of dry wood will be as follows : 

(1) To lieat 1 pound of dry wood from 0° to 140° = 0.33 

(specific heat of wood) X 140 46.2 B.t.u. 

(2) To heat 0.6 pound ice 0° to 32°, 0.6X0.5 (specific 

heat of ice) X 32 9.6 B.t.u. 

(3) To melt 0.6 pound ice=: 143 X 0.6 85,8 B.t.u. 

(4) To heat 0.6 pound of water from 0° to 140° = 

0.6 X 140 84.0 B.t.u. 

Total heat required 225.6 B.t.u. 



COMMON PRACTICES IN DRYING 71 

Turning to Table IX, on page 245, it is found that 
for air entering saturated at 140° and heated to 158°, 
if it leaves the lumber also saturated, the minimum 
possible heat required is 1119 B'.t.u. per pound of water 
from 59°, or 1037 from 140°. To evaporate 0.3 pound 
from and at 140°, therefore, requires 311 B.t.u. As a 
matter of fact, the air will have to leave the lumber 
considerably less than saturated, which will require a 
greater heat consumption, probably at least one-fifth 
more, or 373. Summing up the several heat require- 
ments, to evaporate 0.3 pound of moisture will 
require a minimum consumption of 226 -\- 373 = 599 
B.t.u., or 1997 B.t.u. per pound evaporated. 

Since the outside temperature in this case is much 
colder than before, namely 0° F., it is probable that 
the radiation from the kiln walls will considerably ex- 
ceed the latent heat from the evaporated moisture and 
it would therefore draw directly upon the heat supply. 
This is a factor which can not be accurately worked 
out, however, and there will be other losses in leakages 
through the doors and cracks and through the floor. 
But for the sake of pursuing our system of estimating 
let us see how it can be figured out. 

The same sized kiln as before will hold the lumber, 
having an outside dimension of approximately 
20 X 30 X 115 feet. The superficial area, exclusive 
of floor, will be 5810 square feet. If tlie floor is cement 



72 THE KILN DRYING OF LUMBER 

or sand and inclined to be damp it will transmit at least 
as much heat as the roof, so we will count it as the roof, 
giving a total area of 8110 square feet. The tempera- 
ture in the inside may be taken as an average of 140° 
and 158° or 149°, the outside being 0° Fahrenheit. 
Turning to the table on page 96, it is found that the 
heat loss through a brick wall twelve inches thick is 
about 0.31 B.t.u. per square foot per hour per degree 
difference in temperature. Therefore, the loss per 
hour from the kiln would be 0.31 X 149° X 8110 = 374,- 
601 B.t.u. In six days this would total 374,601 X 144 = 
53,942,544 B.t.u. Let us see now how much heat is 
required for the lumber. 50,000 board feet == 4167 
cubic feet. In one cubic foot of green birch there are 
forty-three pounds of dry wood and 0.6 X 43 = 25.8 
pounds of water, of which half is to be evaporated in 
six days, or 12.9 pounds per cubic foot, a total of 
53,754 pounds. The latent heat at 140° F. is 1013. 
There should, therefore, be available for heating the 
kiln walls 53,754 X 1013 = 54,452,802 B.iu., which is 
seen to be slightly more than the amount required. We 
may, therefore, neglect this factor, for a well-built 
brick kiln. 

Now as to the total amount of heat required to heat 
the frozen lumber and evaporate 30 per cent, of the 
dry weight in water in six days. On page 71 it was 
shown that to accomplish this requires for each pound 



COMMON PRACTICES IN DRYING 73 

of dry wood 599 B.t.u. Therefore, the total amount 
required is 4167 cubic feet X 43 pounds X 599 = 107,- 
329,419 B.t.u. It will be safe, however, to increase this 
by 50 per cent, for leakages, making a total of 160,994,- 
000 in round numbers, or at the rate of 1,118,015 B.t.u. 
per hour. 

In this case low-pressure steam might be used, but 
it would require an excessive amount of piping. "We 
will, therefore, assume, as in the case of the yellow 
pine, that steam at 60 pounds is to be used in order 
to avoid duplication in our calculations. In this case 
h = 21.84(307-140)1-22 = 11,245 heat units per hour 
per 100 feet of one-inch pipe imparted to the air. There 
will be required, therefore, 1,118,015 -^ 11,245 = 99.42 
hundred-foot lengths of pipe, or 9942 linear feet of 
one-inch pipe. It should be observed that in all these 
calculations only an approximation is possible on ac- 
count of the great number of variable conditions. It 
is always a good plan, therefore, to have an excess 
of heating capacity. Furthermore, the drying of the 
free water is assumed to take place at a uniform rate, 
which is not the case. It is interesting in the last 
example where 50,000 feet of green birch timber is to 
be dried in six days to its fiber saturation point, from 
a temperature of 0° F., that 4167 cubic feet X 25.8 
pounds = 1,076,088 pounds, or over 500 tons of ice 
have to be melted before drying even begins ! 



74 THE KILN DRYING OF LUMBER 

Condensing Pipes. — The amount of condensing sur- 
face required in a kiln which is tightly closed and 
dependent upon the condensers for removal of the 
moisture is an indeterminate quantity. It depends 
somewhat upon the amount of moisture to be removed 
per hour, the temperature of the walls of the kiln, the 
amount of circulation of the air, the position in which 
they are placed, the temperature of the condensing 
water, the tightness of the kiln, the temperature of the 
drying, and how much live steam is used at the green 
end or during the beginning of the drying operation. 
It is always safe to have an excess of condensing sur- 
face, since the action can at any time be reduced as 
much as desired by throttling the water flow and thus 
increasing the temperature. Moreover, less water is 
required if the condenser is large than if it is too small. 
If the condenser is too small it is impossible to remedy 
the difficulty by increasing the water flow, since when 
the water leaves the pipes at approximately the same 
temperature as it enters, the limit of action has been 
reached, and an excessive amount of water is required. 

The side walls of the kiln, if it is a separate build- 
ing, the roof, and the doors act as condensers, par- 
ticularly in cold weather, and greatly augment the action 
of the water pipes. If the temperature of any sur- 
face of the kiln is below the dew point, condensation 
wiU occur thereon and drops of water will fall from 



COMMON PRACTICES IN DRYING 75 

the surface just as from a pipe condenser. The con- 
denser should be made up in a continuous coil, or in 
several independent coils, and not with headers or dis- 
tributing pipes. For a kiln twenty feet wide and four- 
teen feet high above the heating pipes from twelve to 
sixteen one-inch pipes placed on both side walls are suf- 
ficient for low-temperature operation 110° to 140°. For 
higher temperatures less pipes are needed. In figur- 
ing on the quantity of water necessary, the tempera- 
ture of the leaving water may be taken as approx- 
imately at the dew point of the air within the kiln. The 
heat extracted must be at least twice, and it is safe to 
figure upon three or four times the latent heat of the 
water evaporated from the lumber in a given time. To 
explain: Suppose a small compartment kiln holding 
10,000 feet of green lumber at 60 per cent, moisture. 
This is to be dried to 5 per cent, in fourteen days. If 
this is red gum at 4400 pounds per M: it wiU weigh 
about 2888 pounds when dry, and the total loss of 
moisture will be 15,120 pounds, or, roughly, about 15,- 
120,000 heat units. This would be 45,000 heat units 
per hour. Assuming a required temperature of 130° 
and 60 per cent, humidity during the first part of the 
drying, temperature of entering water 60°, its maximum 
leaving temperature should not exceed 112°, the re- 
quired dew point (although it may be any amount lower 
than this). Thus the heat absorbed by the condenser 



76 THE KILN DRYING OF LUMBER 

would be 112° — 60° = 52 heat units per pound of 
water circulated. There are 45,000 heat units per hour 
in the evaporated moisture, 45,000 -i- 52 = 862 pounds 
of water. But a lot of heat is likewise extracted from 
the air. Taking three times this amount as suggested 
above gives 2586 pounds of water per hour as the 
minimum quantity necessary at 60° Fahrenheit. If the 
walls and roof are cold, a considerably less quantity 
than this would be needed. 

In the case of the humidity regulated kiln, with 
water sprays on either side in place of pipe condensers, 
less cold water than this is required for the first part 
of the run, but the total amount of warm water recir- 
culated is slightly more than this. A very much greater 
circulation of air is produced by the water sprays than 
by the condensers, however. 

Do not use galvanized pipe for condensers in hard- 
wood dry kilns, as the acid fumes will destroy the 
zinc coating quicker even than plain iron. Paint 
the pipes with a good asphaltum varnish or black 
baking japan. 

Doors, Bails, Trucks, Fire Protection and Acces- 
sories. — Most dry kiln manufacturers supply all the 
apparatus necessary for a complete installation. 
Where rails are used these are usually supported upon 
cast iron or wooden posts, to which also the heating 
pipes are fastened. In progressive kilns, three rails 



COMMON PRACTICES IN DRYING 77 

is a common practice, one in the middle and another 
on either side. The trucks to hold the lumber in this 
case are usually arranged for ''cross piling'*; that is, 
with the boards running crosswise of the kiln. Some- 
times a complete platform is built up of structural 
steel with six wheels to run on the three rails, but more 
often three separate bunks are used, built up of two 
channel irons, four to six feet in length, with two wheels 
between, and the lumber is piled directly upon these 
bunks. This arrangement is for convenience of hand- 
ling the trucks in returning them back again from the 
dry end of the kiln to the loading or green end. The 
journals of the wheels are generally lined with roller 
bearings to reduce friction. 

Sometimes four rails are used and two trucks are 
run into one kiln side by side. In this case the lumber 
is generally ' ' end piled, ' ' that is, with the boards placed 
lengthwise of the kiln, and solid four-wheeled struct- 
ural steel trucks are used. 

In most progressive kilns it is necessary to have the 
rails on an incline to facilitate the movement of the 
cars from the green towards the dry end. A slope of 
2.5 per cent, is common. In some cases this is too 
steep and a slope of 1.4 per cent, or 1 inch in 6 feet is 
found sufficient. 

There are many forms of dry kiln trucks which 
have been patented. Some of these are arranged for 



78 THE KILN DRYING OF LUMBER 

stacking the lumber edgewise, so that the air can pass 
freely through the pile in a vertical direction. A diffi- 
culty with some of these is that no provision has been 
made for keeping a pressure on the lumber after it 
has begun to shrink. Eecent forms of edge stacking 
trucks have a device such that the weight of the load 
of lumber itself brings a lateral pressure to bear con- 
stantly by means of auxiliary bars, thus automatically 
taking care of the shrinkage. There are two different 
forms of this apparatus where the weight of the load 
is utilized to produce the pressure. In yet another 
form the result is accomplished by means of springs 
and cams. These trucks are called automatic shrink- 
age take-up edge stacking trucks. Various systems of 
loading lumber on edge are in use. In one form the 
entire truck is pivoted horizontally and may be swung 
around from the horizontal to the vertical positions, 
and back again. In another the truck is run on to a 
''turn table'* which is tipped by gearing so that the 
boards are loaded in an inclined position, and when 
the load is clamped fast the truck is turned back to the 
vertical position again when it is ready to run into the 
kiln. An automatical loader is also used in connection 
with this truck, so that the lumber is fed directly from 
the mill chains on to the kiln truck, without manual 
labor. Unless edge stacked lumber has some form of 
automatical shrinkage take-up it is sure to warp and 



COMMON PRACTICES IN DRYING 79 

twist in drying, since even if packed very tight when 
placed in the kiln the pile will become loose as soon as 
it dries and begins to shrink. 

Various other forms of holders are used for special 
purposes as staves for pails, small cribbed material, 
etc. In some cases small blocks are dumped on to the 
kiln floor, hopper fashion, in loose piles. 

In many kilns, particularly in those of the compart- 
ment form, no trucks are used, the wood to be dried 
being piled by hand directly in the kiln. 

The door of a dry kiln is an important considera- 
tion, and often badly neglected. Unless the door is 
easily operated, fits air-tight and is a good insulator, 
satisfactory results can not be expected of the kiln. 
A tumbledown door means lost heat and many a lost 
temper, as well as waste labor and inability to main- 
tain uniform moist conditions within the kiln. 

Doors are constracted of wood, of asbestos with 
wood frame, or of canvas. The wooden or the asbestos 
doors are made to slide horizontally as a barn door, 
to lift up, like a stage curtain, and sometimes to fold 
outwardly. One of the very best dry kiln doors slides 
horizontally and is operated by a separate sling frame 
device, which latter runs on a horizontal track above 
the door. This device is moved over the door, and by 
the motion of a single lever the door is clamped, lifted 
oif of its sockets, and swung free (Fig. 15). It may 



80 THE' KILN DRYING OF LUMBER 

then be shoved to one side. It is replaced in the same 
manner and as the lever is released the door drops 
down into its sockets, which are inclined so as to force 
the door tightly against its sills by its own weight. 
This apparatus is patented and known as the Hussey 
Dry Kiln Door Carrier. The doors may be made of 
wood or built up of asbestos framed in wooden lattice. 

Another form of door which gives excellent service 
consists of a double canvas curtain with an air space 
between of about one foot. It is, in fact, two entirely 
separate curtains which are rolled up by means of 
ropes and are so arranged that when they are let down 
the edges may be tightly clamped against their sills by 
means of boards. The main objection to canvas is its 
lack of durability (Fig. 15a). 

There is also an articulated steel curtain door, such 
as is used in warehouses. While it has the advantage 
of being fireproof and durable, it has a great disad- 
vantage in its heat conductivity. II is not generally 
used. 

Forms of Piling. — Probably the commonest form 
in which lumber is arranged for kiln drying is by ''flat 
piling," the boards or planks being laid in horizontal 
layers. Small strips of wood are laid between the 
layers running at right angles to the boards, usually 
these "stickers" are planed to even thickness of about 
three-fourths to one inch for inch lumber and one and 




Fig. 15. — Hussey door carrier. (Courtesy of National Dry Kiln Co.) 




Fig. 15a. — Double canvas doors. 



COMMON PRACTICES IN DRYING 81 

a half to two inches for two-inch planks. The distance 
they are spaced apart depends upon the kind of lumber, 
and the method of piling. For pine lumber they are 
spaced from four to six feet apart, but for hardwoods, 
especially woods inclined to warp, they are placed 
every three feet or even every eighteen inches. The 
stickers should be narrow, so as to cover as little sur- 
face as possible. They are commonly made square, 
though sometimes they are two or three inches wide. 
The use of narrow boards placed crosswise of the pile, 
for stickers, such as is often done in air drying, is bad 
practice for kiln drying and is not usual, since they 
cover up too large an area of the boards. The stickers 
should be dry, and not rotten or stained, nor of a wood 
likely to discolor the boards. Pine, fir, Douglas fir, and 
spruce are excellent woods for stickers. In flat piling 
it is usual to leave a space of at least an inch between 
adjacent boards. Sometimes the boards are staggered, 
that is, placed so that boards and spaces come alter- 
nately one above the other, and sometimes the spaces 
are arranged to come vertically above one another so 
that they form vertical chimneys in the piles. Some- 
times the piles are arranged so as to have one to three 
or more vertical chimneys half a foot to a foot in width 
extending the height of the pile or only half the height. 
Again, tent-shaped openings are left in the center of 
the piles. 



82 THE KILN DRYING OF LUMBER 

A common height of pile is from eight to twelve 
feet ; the length depends upon the length of the lumber, 
and the width may vary from four to six feet. If the 
boards are placed in the pile so as to run crosswise of 
the kiln, the method is known as ''cross piling'' and 
when they run lengthwise of the kiln the term ''end 
piling" is used. As a rule cross piling is used in pro- 
gressive kilns and end piling in compartment kilns. 
A common amount to be placed on a single truck is 
from 3000 to 4500 board feet, but it may be almost any 
amount. 

There is, in fact, as little uniformity in methods of 
piling as in methods of drying. The best methods to 
use in various types of kilns will be discussed in Chap- 
ter VI on the ''Circulation in Dry Kilns." 

Another form of piling is with the boards placed 
edgewise, the stickers running vertically. In this sys- 
tem the boards touch one another, edge to edge. It 
requires a specially designed truck to hold the lumber, 
and unless tliis is arranged to bring a continual lateral 
pressure upon the lumber, it is sure to warp as it 
shrinks. Three or four designs of automatic shrinkage 
take-up trucks have been patented and are occasionally 
used. Though this method of piling is seldom met 
with, its use appears to be increasing in favor, 
especially for Douglas fir and longleaf pine. Special 
stacking machines are generally used where edge 
stacking is practised. 




a^yin^g?nti^^if:^'a^nHuI^"^f^Ree^ "-fl"^^^ ^^^^ '^""'^^l"^ P"'"^ f- kiln 

Note central flue. """icu^J itegulated Kiln. The car contains about 11,000 blanks. 




Fig. 17. — The disastrous effect of improper methods of kiln drying black walnut gun 

stock blanks. 



COMMON PRACTICES IN DRYING 83 

Another form of piling, which has been nsed with 
great success in the author's Water Spray type of kiln, 
is inclined piling (Fig. 44). Here the lumber is sloped 
crosswise of the pile, the stickers being inclined, the 
boards being horizontal as regards the length of the 
pile. The arrangement is such that the air is made to 
pass through the pile in the downward direction, as 
it is cooled by the evaporation and becomes denser in 
passing through the pile. The slope of the pile de- 
pends upon convenience, but should be as great as 
possible for the best effects: a slope of one foot in 
seven gives a good draught. This form of pile re- 
quires no special machinery and may be easily handled 
as a flat pile. The piles should be kept as narrow as 
possible, not over four feet wide for best results, and 
the boards may be placed so as to touch one another 
edge to edge. 

Formed material, such as gun stock blanks, may be 
arranged to advantage in sloped piles of this kind. 
Figure 16 shows such a pile about sixteen feet wide and 
eight feet high containing about 11,000 black walnut 
green wood gun stock blanks. The open spaces, be- 
tween adjacent blanks, due to the uneven contour of 
the material, amounting to about two and a half inches 
at the widest part, still further facilitates the circula- 
tion, allowing the air to move downwardly in a diagonal 
direction through the pile. The blanks are two and 



84 THE KILN DRYING OF LUMBER 

a half inches thick, and abont one-inch stickers are used. 

Shingles are usually bundled, and the bundles stood 
flatwise or edgewise on the trucks, thus leaving many 
horizontal or vertical flues through the piles. Cooper- 
age staves are arranged in a somewhat similar man- 
ner, or are piled in the kiln on end. Smaller staves for 
pales are sometimes cribbed. Laths are bundled and 
the bundles laid horizontally on trucks. Shoe lasts and 
smaller material are usually cribbed. 

End Coating. — Except with the very valuable woods 
it is not the usual practice to paint the ends of lumber 
placed in the dry kiln, chiefly on account of the expense. 
"Where the lumber is sorted into even lengths a good 
practice is to have the end stickers come flush with the 
ends of the boards. As wood dries twenty to thirty 
times as rapidly endwise as it does across the grain, 
it is always desirable to coat the ends with some im- 
pervious, moisture-resisting material. This is espe- 
cially true in the case of small shaped blanks, where 
end checking is a serious defect. Black walnut gun 
stocks are always so coated (Fig. 17). A suitable coat- 
ing must be impervious to moisture, must not melt off 
at the temperature used in the kiln, must adhere to 
the green wood, and must not be too brittle when dry. 
A good coating consists in a mixture of black baking- 
japan and lamp black, to which a little linseed oil is 
added to prevent its chipping. Thin with turpentine 
for applying. 



COMMON PRACTICES IN DRYING 85 

Another excellent coating consists of 

Rosin (good quality) 100 parts by weight. 

Lamp black 7 parts by weight. 

Linseed oil 7 to 10 parts by weight. 

The rosin should be melted at as low a temperature 
as possible, the oil then added, and the lamp black 
thoroughly stirred into the sticky liquid. On no ac- 
count should the mixture be allowed to boil, since that 
will make it froth and the coating will be full of air 
bubbles and not impervious. Keep well stirred in a 
suitable kettle and dip the ends of the sticks at 220° to 
240° Fahrenheit. The coating, when dry, should be 
perfectly smooth and shiny and an eighth to a quarter 
of an inch thick. Do not use paraffin for the dry 
kiln, as it melts at a very low temperature and the 
rosin mixture can not be successfully applied over 
paraffin. A trace of paraffin will cause the rosin mixt- 
ure to melt at a very low temperature. 

Another mixture which has been found to give ex- 
cellent results is the following : 

30 per cent, of 165° C. Pitch 
70 per cent, of Rosin 

Apply at temperature of 400° F. 

May be brushed or dipped. 

Fire Protection. — Modern moist air kilns are much 
safer from fire than dry air kilns. This is largely due 
to the reduction of the amount of oxygen present pro- 
duced by the high percentage of moisture. To be con- 
vinced of this statement one needs only to carry a 



86 THE KILN DRYING OF LUMBER 

lighted lantern into a moist kiln or to attempt to light 
a match. If this humidity is above 70 per cent, the 
lantern or a candle is likely to be extinguished and it 
will be exceedingly difficult to light a match. Where 
steam jets are available or water sprays are used the 
fire hazard is almost negligible. The condenser or 
spray-humidity regulated kiln is the safest kind and 
there is very little risk involved. In ventilated kilns 
particularly, where no attention is paid to humidity, the 
fire risk sometimes runs very high. Woodwork which 
has been subjected to the high temperatures usually 
prevailing in such kilns for a long time gradually 
becomes charred or partially distilled, and is very 
easily ignited. Spontaneous combustion is not uncom- 
mon under these conditions. In a bulletin on Lumber 
and Lumber Drying, issued by the National Fire Pro- 
tection Association in 1914, it is stated that the com- 
mon type of steam-heated dry room has an average 
fire life of but a little over five years. The use of the 
forced draught is said to be a most hazardous type 
and is apt to develop serious fires. Eeferring to the 
use of wooden timbers for track supports and wooden 
piles buried in the earth below the floor, the bulletin 
says: *'A great many fires have occurred in kilns so 
built, the fire having been caused by the result of the 
gradual carbonization of the wood from the heat in the 
kiln. When it becomes charcoal the subsequent heat 
and moisture cause spontaneous ignition." 



COMMON PRACTICES IN DRYING 87 

Again: ''Lumber drying constitutes the principal 
hazard of woodworking factories. In furniture fac- 
tories dry kilns are the source of 37 per cent, of all 
fires and in saw and planing mills 10 per cent. An 
average of all woodworkers gives 21 per cent, of all 
fires as originating in the dry kilns." 

The causes of fires in dry kilns are given in the 
bulletin referred to — ^unknown, 56 per cent. ; known, 44 
per cent. — as follows: 

Per cent. 

Steam pipes in contact with combustibles 30.3 

Sparks entering kiln 28.8 

Overheating 12.2' 

Oily material igniting 10.6 

Fans or blowers 4.6 

Defective flue from kiln 3.0 

Miscellaneous 10.5 

100.0 

Automatic sprinklers fed by adequate and inde- 
pendent water supply installed according to approved 
methods under direction of the inspection department 
of the National Fire Protection Association, having 
jurisdiction, are said to form the only thorough and 
reliable protection against fire. Edge stacked lumber 
is recommended as safer on account of the accessibility 
of the water from the sprinklers. The value of steam 
jets as a fire extinguishing agent in lumber kilns is 
considered as doubtful. Evidence, however, as to their 
actual efficacy is lacking. When a kiln can be tightly 
closed it is often possible to smother a fire without the 
use of any other means. 



88 THE KILN DRYING OF LUMBER 

Construction and Costs of Dry Kilns and of Drying 
Lumber. — The efficiency of a dry kiln must be con- 
sidered with respect to two considerations, namely, its 
(1) ability to accomplish the results desired and (2) 
the consumption of heat required in this accomplish- 
ment. The first may be termed its operative efficiency 
and the second its mechanical heat efficiency. There 
are cases when the first consideration alone will deter- 
mine the choice of a dry kiln, as in the case of expensive 
woods or formed materials which must necessarily 
be dried and which is very difficult of accomplishment, 
or else where there is an excess of steam available 
which would be wasted otherwise. On the other hand, 
there are cases where the second item, the mechanical 
heat efficiency, alone controls the situation. The latter 
is the case principally with very cheap material which 
is easily dried and which is usually done wholly to save 
shipping weights. In this latter case the dry kiln may 
be looked upon simply as an apparatus for evaporat- 
ing water in the quickest manner possible and with 
the least consumption of heat. Ordinarily, however, 
both factors come into consideration, and the problem 
of the selection of a dry kiln, other things, such as 
costs of construction, durability, operating expenses 
(other than heat consumption), rate of drying, adapt- 
ability to varying conditions, and fire risks, being equal, 
resolves itself into the question of the minimum heat 
consumption for the best results obtainable. 



COMMON PRACTICES IN DRYING 89 

It is very evident, moreover, that a design of dry 
kiln which might give the highest heat efficiency for 
the best drying results for one set of conditions might 
prove the reverse for a different set of conditions. For 
example, a system which might prove most efficient 
for drying at a high temperature and high humidity 
might prove relatively inefficient where a low tempera- 
ture and low humidity were required. 

It is, therefore, by no means a simple problem to say 
what design of dry kiln will prove most efficient from 
the heat standpoint for any given set of conditions. 
Many experiments have been conducted by the Govern- 
ment in the development of the first requirement, 
namely, a kiln design which will accomplish the best 
results in drying, for any given problem, but little 
definite knowledge exists as to the most efficient kind 
from that standpoint for given requirements. 

Previous Work. — So far as has been determined, 
there is nothing whatever in literature of a definite 
nature on this subject. Some data of a purely theo- 
retical nature have been worked out by Hausband, 
" Drying in Air and Steam," and by the author in a 
Forest Service bulletin, " Theory of Drying and Its 
Application." Data are also available for most of the 
runs which have been made in the Forest Service "Water 
Spray kiln at Madison, Wisconsin, as to the amount of 
heat consumed in the water, but only a few measure- 



90 THE KILN DRYING OF LUMBER 

ments have been made of the total heat imparted to 
the kiln by the heating pipes and used up through 
radiation, leakages, and evaporation. (The heat of 
evaporation is included in the spray water, or con- 
densing water consumption. ) Further experiments are 
being undertaken by the Government at Madison, Wis- 
consin, with the object of determining the heat con- 
sumption by various methods of drying, under similar 
conditions. 

The reason there is so little knowledge available 
on the heat consumption in commercial kilns is doubt- 
less due to the loose manner in which they are usually 
operated and also to the fact that as a rule the steam 
consumption is of no consequence, low-pressure exhaust 
steam from the engine frequently being used to supply 
the kilns which would otherwise be wasted, or where 
high-pressure steam is used, there has been an un- 
limited supply of fuel in wood trimmings and sawdust. 

Where the exhaust steam from the engine or a 
pump is turned into the heating pipes of the dry kiln, 
an immense saving of heat is secured thereby. The 
steam engine fails to utilize the larger portion of the 
heat contained in steam, namely the latent heat. It 
uses only the heat of expansion of the steam, and the 
steam leaves the engine still in the form of vapor. 
Where this is used for heating, however, there be- 
comes available at once approximately 1000 British 



COMMON PRACTICES IN DRYING 91 

thermal units of heat per pound of steam, due to its 
condensation, and the heating fluid is discharged from 
the system through the steam trap or exhaust pump, 
no longer as vapor, but in the form of water. Thus 
the same amount of steam may be made to do double 
duty, yielding its energy of expansion in operating 
the engine and its latent heat in heating the kiln. The 
water coming from the kiln will still be hot, at the 
boiling temperature, and may be used for the feed 
water for the furnace. It may even be desirable to 
carry a considerable back pressure on the engine of 
seven to twelve pounds in order to secure sufficiently 
high temperature in the heating pipes in the kiln. For 
high temperature drying it is generally necessary to 
use high-pressure steam in order to produce a reason- 
able amount of radiation, as otherwise an excessive 
amount of heating surface would be necessary. 

It is not necessarily the cheapest kiln which is the 
best investment, or the most economical in the long 
run. The choice of the building material and of the 
kind of construction will depend largely upon the per- 
manency of the operation. If the kiln is for a tem- 
porary undertaking, such as for a temporary mill work- 
ing on a limited supply of lumber, a cheap wooden 
structure would probably be the most economical, since 
wood construction is the best insulator, and if the kiln 
simply lasts out the allotted time, that is all that is 



92 THE KILN DRYING OF LUMBER 

necessary. In such a situation there would probably 
be no sales value to the plant after the operation is 
completed, even if built of permanent material. A 
cheap wooden structure can be built to last from four 
to six years. Well-built wooden kilns may last twenty 
or thirty years if properly taken care of and repaired, 
but they are always a fire hazard. 

When a kiln is built in connection with a perma- 
nent operation, as at a wood-working factory, it is 
desirable to build of permanent masonry construction. 
The following materials are in use for dry kilns : Wood, 
studding sheathed on both sides ; wood, solid crib con- 
struction, made of two-inch plank laid one on top of 
the other, thus making a wall of solid wood six or eight 
inches thick ; wood, studding, sheathed inside and with 
metal lath and plaster outside ; brick, solid, or with air 
space; concrete, solid; hollow concrete block; hollow 
tile ; sheet iron, two plates filled between with mineral 
wool. 

From the heat economy standpoint the insulation 
of the kiln is of great importance. Experiment has 
shown that the loss of heat through radiation from the 
roof, walls, and floor, and through leakages may, in 
cold weather, exceed the amount necessary to evaporate 
the moisture from the lumber. Ordinarily, however, 
the latent heat of the moisture evaporated from the 
lumber and the heat in the spent air as it leaves the 



COMMON PRACTICES IN DRYING 93 

pile is more than sufficient to supply these losses. This 
is largely owing to the reason that the heat made latent 
through the evaporation of the moisture from the wood 
is available for helping heat the kiln walls, the evap- 
orated water acting merely as a carrier or agent for 
carrying the heat from the steam pipes to the kiln 
surfaces or to the condensers. If the kiln is properly 
arranged the ceiling and side walls act as condensers, 
and the greater the radiation from the walls the less 
the required capacity of the condensing pipes or spray 
water. 

There is a great difference in different materials 
and kinds of construction in their resistance to heat 
transmission. Solid dry wood is one of the very best 
insulators, and standard wall construction consisting 
of studding, double sheathing and plaster is also ex- 
cellent. Of fireproof materials, solid stone and sand 
concrete is the poorest insulator and standard hollow 
hard burned tile is the best. Brick with air space is 
intermediate. Stone is from 50 to 100 per cent, more 
conductive than brick. 

A great deal depends upon the weather. In north- 
ern climates in wet, windy weather, or during very low 
temperatures and high winds, the heat losses are enor- 
mous. In mild freezing, windy weather a small brick 
kiln with hollow wall and concrete roof sixteen feet 
high, eight feet wide, and eighteen feet long, heated to 



94 THE KILN DRYING OF LUMBER 

160° to 180° F. inside, with high circulation, required 
2500 pounds of steam per day, or an equivalent of 
three boiler horsepower when empty, merely to supply 
radiation and leakages. Where there are a number of 
kilns built together with common walls, the loss will be 
much less than with a single kiln. 

In a brick kiln fifteen and a half feet wide, thirteen 
feet above the pipes, and eighteen feet long, with an 
outside temperature of 70° and inside 114° and sat- 
urated air, it required fifty-one pounds of steam per 
hour to supply radiation and leakages. In another 
test at 132° and 96 per cent, humidity it required 46.8 
pounds per hour. 

In a commercial kiln 107 feet long, holding twelve 
cars and 53,282 feet of inch oak, time of drying 21 
days and 14 hours = 518 hours, the total steam con- 
sumed was 127,600 pounds = 246.3 pounds per hour. 
The approximate amount of water evaporated was 
41,093 pounds; average temperature 158° F. Ap- 
proximately 3.08 pounds of steam were used for each 
pound of water evaporated. This was a condensing 
kiln, but the material of which it was constructed and 
other data are lacking. 

Besides the heat loss there is another very impor- 
tant factor in the material of which the kiln is built 
and that is its absorptive capacity. It is very difficult 
to hold a high humidity in a kiln with absorption walls. 



COMMON PRACTICES IN DRYING 95 

The moisture passes into the walls from the air very 
rapidly, transfuses through them, and is evaporated 
from the outer surface. This also adds considerably 
to the heat loss, as the evaporation of a moist wall is a 
powerful cooling factor. For this reason it is always 
desirable to thoroughly coat the inside of a dry kiln 
with a waterproof paint. High melting asphaltum 
varnish or black baking japan is suitable for this 
purpose. Cement or concrete is the most absorbent 
of the building materials and unless thoroughly coated 
with moisture-resistant paint, or composed of a damp- 
proof mixture, it is unsuitable for a moist air kiln. 
A cement or a sand floor is also highly absorbent, and 
a kiln built on sand, without flooring, will lose its 
humidity very rapidly and is also harder to heat than 
one with an impervious bottom. 

The table on page 96, averaged from a number of 
reliable data, gives the heat conductivity of various 
kinds of walls, in British thermal units per square foot 
of surface per hour per degree difference in 
temperature. 

Figures given by different authors vary so greatly 
that no great reliance can be placed upon any one 
figure. Glass conducts from ten to fifteen times as 
much heat as dry wood, and sheet iron radiates about 
10 per cent, more than glass. 

In regard to cost of construction, this will vary, 



96 



THE KILN DRYING OF LUMBER 





Solid 
brick 


Slag or 

cinder 

concrete 


SoUd 
concrete 

(stone 

and 

cement) 


Concrete 

with 

air 

space 


For 

stone 

use 


Wall 8 inches thick 


0.43 
0.31 
0.26 
0.23 
0.20 

0.34 
r 0.30 
■ (.25 to 

.44) 

0.41 

1.1 
.59 

0.2 
/ 0.08 
\to 0.17 

0.45 

0.26 


.29 


0.89 
0.77 
0.67 
0.59 
0.54 


.54 

.50 

.46 

.421 

.39" 


1.5 to 2 


Wall 12 inches thick 


times 


Wall 16 inches thick 


values 


Wall 20 inches thick 


given 


Wall 24 inches thick 


for 


Stud partition, lath and plaster 
two sides 


brick 


Walls of average wooden house . 

Wooden door 1 inch thick 

Window glass, single 




Window glass, double 




Concrete floor on brick arch . .,. 

Wooden beams planked over as' 

floor 




Single %-inch wooden floor, no 
plaster beneath 




Single %-inch wooden floor, 
with plaster beneath 





of course, with local conditions, such as cost of labor 
and materials. The kind of kiln will also make some 
difference. 

In general the kiln may be divided into the fol- 
lowing items : Building proper, foundation, walls, roof ; 
heating apparatus; doors, rails, trucks, etc. About 
half the total cost of a brick building may be allowed 
for the apparatus and accessories. 

A recent estimate (December, 1916) on a plain 
kiln room 100 feet long, twelve feet high, and twenty 
feet wide, with back wall but with front open, is given 
on the following page : 



COMMON PRACTICES IN DRYING 97 

Foundation, concrete and excavation $250 

Walls: 

Wood studding sheathed inside 1" matched hemlock, outside 
1" hemlock covered with one thickness building paper and 

hemlock shiplap 595 

12" hollow tile 700 

12" concrete block 924 

12" brick with air space 1000 

Roofs : 

Frame construction 2'80 

Concrete beam and tile 900 

Reinforced concrete slab 1000 

Steel frame and book tile 800 

Waterproofing Roof: 

Four-ply composition of tar and gravel 80 

The foundation and waterproofing would be the 
same in any case. 

For all wooden construction the cost of the building would be. . . . $1205 

For hollow tile and tile roof 1930 

For concrete blocks and concrete slab roof 2254 

For brick and cement slab 2330 

For heating pipes and apparatus allow - 1200 

Then the total costs are as follows : 

All wood , $2405 

Hollow tile 3130 

Concrete block 3454 

Brick 3530 

Other sizes would be in proportion; where several 
are built in a bank there would be the saving of the 
adjacent walls. 

The following is the actual cost of certain kilns in 
1910. There were two adjacent kilns having one com- 
mon wall 150 feet long, sixteen feet high, eighteen feet 
wide, built of eight-inch hollow tile walls, cement f oun- 
7 



98 THE KILN DRYING OF LUMBER 

dation, hollow tile roof with reinforced concrete girders 
covered with three-inch cinder cement slab overlaid 
with four-ply roofing: 

Building $4900 

Trucks, piping, and apparatus 2918 

Two doors 100 

$7918 

It is estimated that in 1913 this would have cost 
$9900, and at the present time considerably more. 

Cost of Operation. — The consumption of steam in 
well-insulated kilns in operation below 160° F. may be 
roughly taken as three to three and one-half times the 
amount of water evaporated from one-inch green lum- 
ber. For thicker sizes a greater proportion is required, 
and where the drying is carried much beyond the 
6 per cent, of the dry weight, it may be greater than 
this; where very high temperatures or superheated 
steam is used it may run up to as high as four or five 
times the water evaporated. 

As there is frequently an excess of steam available 
or the exhaust from the engine is used for heating 
the kilns, the cost of the steam is frequently not in- 
cluded in estimates of drying. Repairs, deterioration, 
interest on investment, insurance, should, however, be 
included. In some estimates a portion of the over- 
head ("administrative") charges of the business is 
included in the cost of kiln drying. The cost of labor 



COMMON PRACTICES IN DRYING 99 

in piling and handling the lumber through the kiln 
must, of course, always be included. From this it will 
be seen that the actual cost of kiln drying the lumber 
is rather an elastic item, and can hardly be given in 
general terms. Estimates for drying hardwoods vary 
from 50 cents to $7.50 per M. A firm has been drying 
lumber for the trade at the following rates: Poplar 
$2.50, oak $3.25, for inch lumber, adding 25 per cent. 
for each additional quarter inch in thickness. Evi- 
dently this could be done at a profit or the concern 
would not have done it for the trade. No guarantee 
of any kind, however, was given, and considerable 
checking frequently occurred. Where a manufacturer 
does his own drying in connection with a saw mill or a 
woodworking plant, a reasonable expense for kiln dry- 
ing one-inch hardwoods would be between $1 and $1.50 
per M. An estimate given by a dry kiln manufacturer 
for condensing kilns is $1.04 per M in 1914. 

In regard to the costs of handling lumber in the 
kiln over against piling in the yard, this would ordi- 
narily mean piling and unpiling once. With suitable 
arrangements there would be no additional piling 
necessary ; that is, where the plant is so arranged that 
the lumber is loaded upon the kiln truck directly from 
the chains of the mill and again unloaded from the 
kiln truck, which may have been kept several days in 
the dry shed, directly into the railroad car. Where 



100 THE KILN DRYING OF LUMBER 

the lumber is first piled out in the yard and then loaded 
into the kiln, and again piled in the dry shed, where 
it remains until called for, the cost of the loading and 
unloading must be included in the expense of the kiln 
drying as against air drying. This will ordinarily 
cost from 50 to 70 cents per ^. The cost of air dry- 
ing, not including depreciation, taxes, or insurance, 
nor interest on investment, but covering sorting and 
grading at the mill, handling and loading on the cars 
for shipment in a large oak and gum manufactory in 
the South, is $1.49 per ^. 

Two men can load or unload and sort 20,000 to 
25,000 feet of one-inch lumber per day. With machine 
stackers two men with two machines can stack 80,000 
to 120,000 feet per day. 

General Layout of the Plant. — The arrangement of 
the kilns with respect to the saw mill, power house, 
factory, and yard mill, of course, varies with local re- 
quirements. It can usually be arranged with a little 
thought so as to give a minimum in handling of the 
lumber. The kiln should be near the boiler or engine 
room; there should be provision made for reasonable 
expansion without altering the convenience of the 
system of handling. There should be ample dry stor- 
age room and it will be found convenient to have a 
green loading room under cover as well as an open 
storage to take care of fluctuations in the operating 



COMMON PRACTICES IN DRYING 



101 



Fig. 18. 



SCALE -r* FCET 



OPEN STORAGE 



D H Y 

3333 Tt 



lYARDrriROOiiryoRrr:: 

FUTURE KILNS 



tAiRtejtiniE: 



FACTORY 



operat'q 



BOILER 
ROOU 



FINISHING 



COMPARTMENT SYSTEM. 



OPEN 


STORAGE 


YARDROOM FOR FU 


T 


DE KILNS 


PLAMIM& 




1 1 




■ 




-i— 








H H 














DRY 






GREEN 






















£ 
FINISHlua 


SHED 






SHED 

























■ 












FACTORY 


|0PERATG 


BOILER 
ROOM 




IS 



PRDGRESSIYE SYSTEM 

Fia. 19. 
Fig. 18. — General layout of plant with progressive kilns. 
Fig. 19. — General layout of plant with compartment kilns. 



102 THE KILN DRYING OF LUMBER 

capacities. The accompanying diagrams are sugges- 
tive of a convenient layout of a plant designed to dry 
12,000 board feet per day, assuming fourteen days as 
the length of time necessary to hold in the kilns. 
Figure 18 represents a battery of four progressive 
kilns each eighteen feet wide, ninety-two feet long, 
holding fourteen trucks, with 3000 board feet to each. 
These are ventilated end draught kilns, with lumber 
cross piled, flat. Figure 19 represents a battery of 
fourteen compartment kilns, fifteen feet wide, thirty- 
four feet deep, each holding two trucks of 6000 feet 
each, end piled slant piling spray or condenser kilns. 
The advantages of the compartment kilns are: (1) Less 
handling of the lumber, the trucks remaining stationary 
for fourteen days, whereas in the progressive kilns all 
of the trucks have to be moved every day. (2) Much 
better control of the temperature and moisture con- 
ditions. (3) Accessibility of each entire kiln every 
fourteen days. (4) Various kinds and thicknesses of 
material may be dried at the same time, each in a differ- 
ent compartment. For wooden kilns, insurance require- 
ments may necessitate a different arrangement, with 
separation of the kilns from the rest of the plant. 



CHAPTER IV 

How Wood Dkies; Shkinkage, Wakping, 
Casehaedening 

Geeen wood contains water in two forms, namely, 
as free water; tliat is, as small particles of liquid water 
occnpying the capillary spaces in the wood, as the 
lumina of the fibers and the vessels ; and as hygroscopic 
moisture intimately absorbed by the substance of the 
cell walls. The hygroscopic moisture appears to be 
molecularly distributed between the particles or minute 
lamina of which wood substance is built up, in the 
same manner as it enters into colloidal substances. 
It can be extracted from the substance without chang- 
ing its chemical properties, but the substance begins 
to shrink with the slightest loss of this moisture. More- 
over, it requires an additional amount of heat^ than 
that necessary to evaporate free water in order to 
extract this moisture from the wood substance. In 
other words, the wood substance has an affinity for 
moisture and is capable of condensing a certain amount 
from the vapor in the air. In fact, there is a definite 
relation between the relative humidity of the air and 
the amount of moisture which the wood will retain. 

* 33.7 B.t.u. per pound of dry wood or about 135 B.t.u. per pound of 
water extracted, according to figures obtained by Frederick Dunlap. 

103 



104 



THE KILN DRYING OF LUMBER 



The relationsliip between the relative humidity and 
the percentage of moisture retained is given for cotton 
(pure cellulose) and for several species of wood at 
212° F. by H. E. McKenzie in Figure 20. 

Table VI. — The Fiber Saturation Point for Several Woods asi 
Determined by Compression Tests on Small Specimens. 

Average 
moisture per 
Species Condition of eravitv cent, at fiber 

saturation 
point 



Longleaf pine 

Red spruce 

Chestnut 

LoboUy pine heart 
LoboUy pine heart 
Loblolly pine sap . . 
Loblolly pine sap . . 
Norway pine heart 
Norway pine sap . . 

Tamarack 

Western hemlock . . 

Red fir 

White ash 

Red gum 

Red spruce 

Tamarack 

Chestnut 





No 


Average 


Condition 


of 

tests 


specific 
gravity 
kiln dry 


Green 


40 


.62 


Green 


40 


.38 


Green 


40 


.48 


Green 


24 


.59 


Air dry 


80 


.67 


Green 


72 


.47 


Air dry 


80 


.55 


Green 


121 


.42 


Green 


88 


.44 


Green 


121 


.54 


Green 


144 


.54 


Green 


65 


.58 


Green 


49 


.79 


Air dry 


45 


.52 


Air dry 


120 


.45 


] S u p e r- 


120 


.44 


heated 






[Air dry 


120 


.60 


] S u p e r- 


80 


.61 


heated 






[Air dry 


120 


.48 


j S u p e r- 


120 


.46 


[ heated 







25 

31 

25 

23 

24 

24 

26 

30 

28 

30 

29 

23 

20 

25 
30 to 32 
24 to 25 

30 to 32 
24 to 25 

26 to 27 
22 to 24 



Other phenomena take place when the hygroscopic 
moisture is extracted : The strength begins to increase, 
so that perfectly dry wood may become four times as 
strong as it was when green. The wood substance of 
most species has a limit in the amount of hygroscopic 
moisture which it can contain. This limit is called 



HOW WOOD DRIES 



105 



the "Fiber Saturation Point." It varies from 20 to 
30 per cent, of the dry weight of the wood. See Table 
yi. In rare eases it appears to be as high as 90 per 
cent., since shrinkage begins at this point, for example, 




4- % IZ f6 ZO 

Moisture T^efcent of Xty Wet^ht- 

Fia. 20.— Relationship between relative humidity of the air and percentage of 
moisture retained by cotton and by several species of wood (after McKenzie). C, cotton; 
B, birch; H, hickory; A, ash; T, tulip; S, spruce; P, pine. 



106 THE KILN DRYING OF LUMBER 

in Eucalyptus globulus and a few other euoalypts, and 
it is abnormally high also in some of the southern 
swamp oaks. 

In the living tree the amount of moisture in the 
wood may range from 250 per cent, of its dry weight 
to about 30 per cent. The heartwood of conifers is 
usually near to its fiber saturation point, containing 
about 30 per cent, of moisture, but the sapwood, which 
contains much free water, often has from 100 to 150 
per cent. In the hardwoods sometimes both heart and 
sapwood contain 60 to over 200 per cent, moisture, but 
frequently the sapwood contains more free water than 
the heartwood. No wood in a healthy condition in the 
living tree is drier than its fiber saturation point. Dry 
wood is, therefore, from the physiological standpoint, 
in an abnormal condition, it has never been dry before 
since its creation from the cambium cells of the tree. 

Season of Felling and Moisture. — There is a wide- 
spread opinion that wood in the summer time contains 
more water than in the winter. This is not the case ; 
in fact, during the period of most active growth in the 
spring the tree sometimes contains less water than it 
contained during winter.^ It is true that the ''trans- 



=' Investigation by Hartig in Die Technichen Eigenschaften des 
Holzes by H. ISTordlinger, 1860. Also see investigation by Gabriel Janka 
on " Einwirkung von Siiss- und Salwassern auf die Gewerblichen Eigen- 
shaften der Hauptbolzarten " in Mittbeilungen aus dem Forstlichen 
Verscbuchswesen Osterreichs, Folge xxxiii heft. 



HOW WOOD DRIES 107 

piration current, ' ' or the ^ ' sap ' ' is moving mucli more 
rapidly at the time of active growth. The expressions 
that the *'sap is up" in spring and that it reposes '4n 
the roots** in winter are incorrect. Marked chemical 
changes take place in the sap of the tree between sum- 
mer and winter, but the total amount of water in the 
wood does not change greatly with the season. In the 
winter the food is stored in the sapwood as insoluble 
starch and gums, and in the spring these are reduced 
again to sugars, which are soluble and are carried 
through the living tissues. This is one reason why 
sugar is so plentiful in maples at certain seasons of 
the year. Great changes in internal pressures occur 
within the tree, especially in the early spring when 
the sap is flowing. Actual pressures have been meas- 
ured in maple trees from a partial vacuum of five 
pounds below atmospheric to twenty-five pounds gauge 
above. In one case this extreme variation in pressure 
of thirty pounds per square inch occurred between 
early morning and noon of the same day. This change 
in pressure is what causes the flow and it is partly 
due to changes in temperature.^ 

Probably what changes in moisture do occur are 
confined to the sapwood only. There is considerable 

'Bulletin No. 105, Vermont Agricultural Experiment Station, 
Burlington, 1904, "The Maple Sap Flow," from researches by C. H. 
Jones, A. W. Edson, and W. J. Morse. 



108 THE KILN DRYING OF LUMBER 

difference of opinion as to relative merits of cutting 
timber at different times of year. Probably the most 
common opinion is tbat winter cutting is the best "be- 
cause the sap is down." Some long experienced and 
observing lumbermen, however, state that the summer 
felled timber is the best. 

So far as the amount of water present in the beart- 
wood is concerned there is in reality no difference. There 
is this to be noted, however, when a tree is felled which 
is in full leaf and allowed to remain until the leaves 
shrivel up, practically all of the free water is drawn 
off from the sapwood by the rapid transpiration of the 
leaves. Thus the outer layer of the logs will dry very 
quickly and danger from stain or rot is greatly re- 
duced or eliminated. The heartwood is liot affected 
by this treatment. The same thing is accomplished by 
girdling the standing trees, cutting clear through the 
sapwood. This is a common j ractic^n the teak forests 
of British India, where the trees are girdled one or two 
years before they are felled. 

On the other hand, if the tree is immediately sawed 
ap into logs, the sapwood contains much soluble foods 
which render the logs very susceptible to fungous 
attack, causing stain and rot, during the summer. In 
this case, winter felling is, no doubt, preferable, as the 
logs have a chance to season slightly during cold 
weather, so that when the warm weather favorable to 



HOW WOOD DRIES 109 

fungous growth arrives the surfaces of the logs will 
have dried down below the danger point. 

The butt portions of trees which grow in damp 
ground frequently contain much more free water than 
the tops of the trees or than trees growing on drier 
ground. These butt portions are usually full of shakes 
and may contain 100 to 200 per cent, moisture, whereas 
the tops contain only 30 per cent. Such trees frequently 
have swelled butts. This is particularly true of western 
red cedar, western larch, tupelo, cypress, sugar pine, 
white elm, redwood, hemlock, etc. 

Soaking in Water. — ^Another current belief is that 
long-time soaking causes wood to dry much quicker 
and renders it less subject to warping and "working." 
"W«od which has been buried in swamps is eagerly 
sought after. Wliile there is no direct evidence that 
soaked wood seasons any more rapidly than green 
wood, it seems vpry- probable that long leaching for 
years may reduce tte tendency to check and to warp. 
The tannins, resins, gums, and albuminous materials 
undergo a gradual change under water, and much of 
the deposited materials is gradually leached out. The 
internal stresses also gradually disappear. The ap- 
pearance of driftwood, particularly of logs which have 
long been under water, strengthens the theory. The 
wood appears to become more porous, due to the leach- 
ing out of these substances, but it must require a very 



110 THE KILN DRYING OF LUMBER 

long time, many years for whole logs, to accomplish 
these results.* That is why it is difficult to find any 
exact comparative data. Boiling in water or soaking 
in hot water accomplishes a somewhat similar result 
in a much shorter time. In Japan it is a common 
practice to soak wood for two to five years, before 
using, in large municipal tanks, in a mixture of six 
parts sea water and one part fresh water. The sticks 
are turned and cleaned occasionally. Just how much 
this treatment is responsible for the world-wide repu- 
tation of Japanese wooden articles is uncertain. 

How Wood Dries. — In order to get an understand- 
ing of the phenomena which take place as wood dries, 
let us consider a piece of green lumber containing 75 
per cent, of water, 25 per cent, being hygroscopic and 
50 per cent, free water, and follow the process as nearly 
as we can, step by step. The evaporation must take 
place from the surface and the water in the interior 
must pass from cell to cell until it reaches the surface. 
In order that there shall be any movement of the 
water within the stick there must be a gradient of 
moisture condition or a difference of temperature 
between any two portions. Therefore, the surface 
must be drier than the inside. Just how the moisture 
passes through the wood is not known. It may all 
pass through the ceU walls as hygroscopic moisture, 

* See reference to Janka's researches, footnote, page 106. 



HOW WOOD DEIES 111 

or the free water may pass through the pits in the walls 
by capillarity, as it passes off from the surface. What 
probably happens is this: the free water evaporates 
first from the surface and if the rate of evaporation is 
greater than the rate at which the internal free water 
can pass to the surface, the hygroscopic moisture also 
begins to evaporate from the surface, which then begins 
to shrink and surface check. In this condition the 
columns of free water become interrupted, and a re- 
tardation of the transfusion of free water from the 
inside takes place. This is one phase of ''caseharden- 
ing." On the other hand, if the surface evaporation 
is not too rapid, a continuous flow of the free water 
from the center to the surface takes place, until all of 
the free water has passed off. It is evident that the 
hygroscopic moisture must pass outward only through 
the substance of the cell walls themselves, and not as 
the free water through the capillary spaces. Evidently 
this would be a much slower process, which supposi- 
tion is borne out by the drying curves, which all drop 
off in rate quite suddenly near the fiber saturation 
point. (See Curves, Fig. 31.) The transfusion end- 
wise of the grain is very much greater, probably ten 
or twenty times as rapid as it is across the grain, as is 
evidenced by the end checking of lumber, and it is 
more rapid radially than it is in the direction of the 
circumference or tangentially. This is why quarter- 



112 THE KILN DRYING OF LUMBER 

sawed or edge grain lumber dries more slowly than 
slash-sawed or flat grain. But end drying can not be 
ordinarily counted upon for removal of the moisture 
from lumber and it must nearly all pass out in a direc- 
tion across the grain. This process of transfusion is 
very slow, so that it requires from three months to 
several years for one inch thick lumber to dry in the 
air. I have found that moisture passes from the 
warmer towards the colder portion of a block of wood. 
If it be heated on one side and cooled on the other, the 
water will be driven from the heated side to the cold 
surface. If a hot piece of wood containing some 
moisture be placed upon a cold piece of iron, the moist- 
ure will be very quickly condensed upon the cold metal 
surface. This is a very useful quick test to determine 
whether a piece of wood from the kiln is dry. A freshly- 
sawed end-grain surface should be used for this test. 
The cause of this phenomenon is not certain, but a 
possible explanation is the following : Consider a series 
of cells joined side to side in a row and heat applied 
at one end and cold at the other. The moisture is 
vaporized from the hot side of the first cell and the 
cavity within is saturated with the vapor. The op- 
posite surface being slightly cooler, condensation must 
take place thereon. The condensed moisture then 
soaks through the common wall and is re-evaporated 
in the next cavity and again condensed on the further 



HOW WOOD DRIES 113 

wall. TMs is repeated clear through the series of 
cells until finally the moisture is condensed on the cold 
metal surface or is evaporated in the cold air. If a 
piece of wood containing free water be steamed under 
slight pressure until it becomes heated through to the 
center and then taken out from the steam and placed 
in a cooler atmosphere, the moisture is driven from 
the interior towards the surface so long as there is a 
difference in temperature between the inside and the 
surface. This effect is highly beneficial, but can not 
well be repeated, after the block has once cooled down, 
since the second heating tends to drive the moisture 
back again towards the center. 

A disk three inches thick, cut across a freshly felled 
basswood tree, was split in two parts through the center. 
One part was steamed twenty minutes at twenty pounds 
gauge ; then both halves were stood on edge on a table 
in a warm, dry room. The disk contained 97 per cent, 
moisture when cut. In eight days the unsteamed piece 
had dried to an average of 19 per cent, moisture and 
was full of numerous fine checks or crackles on both 
surfaces, large enough to insert the point of a lead 
pencil. Marks on the surface made with cobalt chloride 
had turned blue, indicating that the surface was dry, 
the inside being still quite moist. The steamed piece 
dried a little slower than the unsteamed, but developed 
no crackles other than two very fine checks, even after 



114 THE KILN DRYING OF LUMBER 

complete drying. Moreover, cobalt chloride marks on 
the surface remained pink long after those on the other 
piece had turned blue, indicating a moist condition of 
the surface even when the entire block had dried to 
considerably less than the fiber saturation point. 
Finally the crackles on the first piece closed up again, 
showing that the interior was drying and beginning 
to shrink. This experiment shows plainly the effect of 
the internal heat in driving the moisture towards the 
cooler portions. Hence the desirability of having the 
lumber heated clear through before drying begins. 

A study of the microscopic structure of wood shows 
why the drying takes place so much more rapidly end- 
wise than across the grain. 

CaseJiardening. — As a green piece of wood dries, 
shrinkage as a rule^ does not begin until the fiber 
saturation point has been reached. If the conditions 
of drying are such that the evaporation from the sur- 
face exceeds the rate at which the water in the center 
transfuses outwardly, the free water on the surface 
will all disappear before the inner water can take its 
place. The water films or columns thus become broken 
with air in between, so that the remaining water has 

° In some woods, notably western red cedar, a collapsion of the cell 
cavities begins to take place as soon as drying begins, which causes the 
stick to appear to shrink at once. This is not true shrinkage, however, 
as it is a totally different phenomenon. Eucalyptus, " blue gum," 
appears to begin to shrink at 80 to 90 per cent, moisture, due to a form 
of collapse. 



HOW WOOD DEIES 



115 



to pass out gradually by seepage along the cell walls 
and further transfusion of the free water from the 
inside is thus retarded. This is the first stage of case- 
hardening. 

As drying then progresses beyond the fiber satura- 
tion point, the outer shell tends to shrink, but is pre- 
vented from doing so by the wet interior. Strong 
tension stress is therefore set up in the outer surface 
with compression in the inner portion. This condition 



^> + + , 




Fig, 21. — First stages in casehardening. The figures represent a disk cut across 
the timber perpendicular to the grain. 

(A) Surface dried below the fiber saturation point and in a state of tenaion. Inte- 
rior containing free water and in compression. Tension stress indicated by minus sign 
( — ); compression by plus sign (+). 

(B) If disk A be immediately sawed by five slits, as shown, it will at once spring 
into this shape. 

(C) If B be now dried in an oven or warm room, the tongues will finally bend into 
this shape and remain so permanently. 

is shown by Fig. 21-A, which represents a disk sawed 
across a plank of wood at this stage. 

If this disk be immediately slotted, as shown at 
Fig. 21-B, by running five parallel saw cuts nearly 
through, and breaking away two of the tongues thus 
formed, the outer prongs will at once bend outwardly 
as shown in the figure. This indicates temporary case- 
hardening, due primarily to a dry surface and wet 



116 THE KILN DRYING OF LUMBER 

interior. If this disk be then placed in a dry room 
where it will dry out, it will finally assume the form 
shown at Figure 21-C. 

Wood is a plastic substance when hot and moist. 
When dry it becomes very much harder, stiffer and 
stronger than when wet. Consequently, when a stick 
of wood is bent while hot and wet, and then dried in 
this position, it will rigidly retain the form into which 
it was clamped when drying. This well-known prin- 
ciple, made use of in wood bending, also applies in the 
case of casehardening. The outer shell, which dries 
first below its fiber saturation point and thus tends to 
shrink, is prevented from so doing by the wet 
interior. As it continues to dry it hardens in this ex- 
panded condition. (See Fig. 21-A.) The interior now 
continues to dry very slowly, and tends to shrink, but 
is in turn prevented from doing so by the hardened 
outer shell which has ''set" in its expanded condi- 
tion. The result is that the stresses within the block 
are now reversed, the outer shell being in compression 
and the inner portion in tension, as shown in Figure 
22-A (compare with Fig. 21-A). If the tensile strength 
of the wood across the grain is sufficient to withstand 
these stresses, the wood will dry in this condition con- 
• taining permanent intemal stresses, but if it is weak 
it will yield to the tension stresses and split open as 
shown in Figure 22-B, usually along the medullary 



HOW WOOD DRIES 



117 



rays. This is the explanation of honeycombing or 
''hollow horning." 

A peculiar physical property which accentuates this 
result of permanent casehardening is the fact that the 
slower wood dries, the greater is the shrinkage. Con- 
sequently, not only does the stress result from the ex- 
panded set condition of the outer shell, but also from 




+ + 


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" k 


t -" 


— it 


t' _ 


— "!+ 






i] __ 


_',f 


T\ — 






- ^x + 


■+ ;- 






Fig. 22. — Final stage in casehardening and honeycombing. This is the condition 
of permanent casehardening. 

(A) Permanent internal stresses remaining after the wood has completely dried. 
Tension stress indicated by minus sign ( — ) and compression by plus sign (+). Compare 
with Figure 21 A and note that the stresses here are completely reversed. 

(B) Honeycombing resulting from the stresses shown in A ; the strength of the wood 
across the grain was insufficient to resist the internal tensile stresses and the fibers have 
separated along the medullary rays. 

(C) If disk A be sawed by fine slits, as shown, it will immediately assume this form 
and bind on the saw. 

the fact that the interior tends to shrink more than 
the shell, due to slower rate of drying. 

The phenomenon of collapse ^ also probably enters 
into this honeycombing in certain species, such as west- 
ern red cedars, causing excessive shrinkage of the in- 
terior portion. 



' See article by the writer, " Problems in Kiln Drying Lumber," in 
the Lumher World Review for September 25, 1915. 



118 



THE KILN DRYING OF LUMBER 



Proof of Stresses in Casehardened Wood. — To 
prove that the stresses exist permanently, as explained 
in the casehardened wood, it is only necessary to take 
a disk A, Figure 22, and slot it, as described before. 
The prongs will at once turn inward, as shown in 
Figure 22-C, frequently with such force that it is diffi- 
cult to remove the piece from the saw. This disk will 





Fig. 23. — End view of a casehardened board — (A) Internal stresses; minus sign 
( — ) is tension and plus sign (+) is compression. 

(B) Form which the board will assume after resawing. 

(C) Test for internal moisture — note the bending outward of the center tongues 
upon exposure to dry air. They will ultimately turn inward as in Figure 22 C. 

remain definitely in this condition so long as it is kept 
dry. 

If the plank was not thoroughly dry when the disk 
was sawed, as shown in C, these prongs will close up 
still further as the disk dries. Compare this figure 
with B in Figure 21, when the stresses were temporarily 
of the opposite kind. 



HOW WOOD DRIES 119 

From this explanation it is perfectly plain why a 
casehardened board will cup when resawed, as in Figure 
23. For the same reason, when articles are manu- 
factured from casehardened wood, they are very apt to 
warp and give trouble. The two inner tongues of the 
slotted disk give an excellent indication of whether the 
interior of the wood is dry. If the interior is not dry, 
the tongues will show little or no tendency to bend on 
the saw, but after remaining in a dry room for a short 
time they will begin to bend outwardly as shown in 
Figure 23-0. 

They will finally, however, bend inwardly as shown 
in Figure 21-C, as the disk dries out. If they remain 
parallel in a dry room it indicates that the wood was 
thoroughly dry (that is, corresponding to the room 
condition) when the disk was cut. The explanation 
of the behavior of these tongues as described above is 
significant. 

In Chapter VI I have stated that drying should 
take place uniformly from the opposite faces and that 
if drying took place from one face only the board or 
stick would tend to cup — ^not with the first dried face 
as the concave surface, as might at first be supposed, 
but just the reverse. The surface which dried first 
will become the convex surface. 

This phenomenon is exactly what takes place in the 
two parallel tongues of the pronged disk, and the 



120 



THE KILN DRYING OF LUMBER 



explanation is as follows: The outer surfaces, being 
exposed to the air, dry first below their fiber saturation 
point, and these surfaces dry in a stretched condition, 
being held by the resistance of the rest of the tongue 
to bending. That this is so is shown by the slight 
bending of the tongues outward (Fig. 23-C). 

The dried surface hardens in this stretched con- 
dition. The rest of the tongue subsequently dries more 



A 




Fig. 24. — Test disks for removal of casehardening. In A the stresses have been 
entirely neutralized and casehardening eliminated. In B, the process has been carried 
too far and the condition has been reversed. 

slowly and shrinks more than the outer surface ; con- 
sequently the curvature ultimately becomes reversed 
and the tongues bend inwardly as shown in Figure 21-C. 
The rapidly dried surface of a board or stick (dried 
from the green condition) therefore tends to be convex 
and not concave. 

This action is often of considerable consequence in 
the drying of wood, especially of material sawed into 
blanks for manufacture after drying. 

Removal of Casehardening. — ^A clear understand- 



HOW WOOD DRIES 121 

ing of the nature of casehardening will, with a little 
thought, suggest a means by which it may be removed. 
In fact, it is not only possible, but entirely practicable, 
on a large scale, to eliminate casehardening, provided 
it has not gone to the extent of causing honeycombing. 
Very recently I discovered the method which I will 
now describe. 

The only condition of permanently casehardened 
material which is of serious consequence is that it 
contains internal stresses. If these can be removed, 
the trouble will be overcome. With a knowledge of the 
distribution of these stresses it is evident that if we 
soften up the external shell containing the compressive 
stresses, so that it becomes plastic, the stresses will 
readjust or neutralize themselves and disappear. It 
has been pointed out that wood substance becomes 
plastic when hot and moist. Therefore, if the case- 
hardened wood be placed in steam or in hot moist air 
for a while, sufficient time only to soften up the external 
shell, the result will be accomplished. The temperature 
necessary probably varies with the different species, 
as some become much more plastic than others at the 
same temperature. 

The time of exposure necessary will vary with the 
degree of casehardening, but it is astonishing how 
quickly the result is obtained, usually in from ten 
minutes to half an hour in saturated air at 180° to 
200° F. One great advantage of this treatment is that, 



122 THE KILN DRYING OF LUMBER 

since the moisture need penetrate only the outer sur- 
face, it very quickly re-evaporates. Moreover, it can 
be done on any scale whatsoever. 

Prohably the simplest way is to place perforated 
steam pipes in the kiln or in a separate room into 
which the truck can be run, arranged in such a way as 
to produce a maximum circulation through the pile of 
lumber. High-pressure steam and plenty of it should 
be available. 

For a couple of truck loads of lumber, each about 
16 X 8 X 5 feet in size, a one and one-half inch pipe 
with boiler pressure steam (eighty to a hundred pounds 
gauge) should be used, having in all about eighty one- 
eighth inch holes properly distributed, and this should 
be opened up full. 

The temperature should be allowed to rise as rapidly 
as possible, and the steam turned off again after the 
proper time, which must be determined by experiment 
for a given kind of material and conditions. Drying 
conditions should then be re-established as soon as 
possible and the load left in the kiln for a day or two. 

For two and one-half inch black walnut at about 5 
to 6 per cent, moisture, I have found that steaming in 
this manner for half an hour, the temperature rising 
from about 130° to 180° during this time, gives satis- 
factory results. In from one to three days* time the 
absorbed moisture will have entirely evaporated and 
the material will be as dry as before steaming. 



HOW WOOD DRIES 123 

No fear need be had of injuring the finest woods by 
this treatment, and it is so simple that it is within 
the reach of everyone who has a dry kiln. There is no 
danger whatever of checking the wood, since the sur- 
face is already in compression, and all stresses are 
relieved thereby and not augmented. 

If the process is carried too far a reversal of 
stresses may be produced. This can be readily de- 
termined by cutting the pronged disk. If stresses are 
reversed, the outer prongs will bend outwardly as 
shown in Figure 24-B. If exposure has been correct, 
the prongs should remain parallel as shown in Figure 
24-A. 

The test disks should not be cut until the moisture 
has re-evaporated. 

Figure 8 shows the ultimate results of caseharden- 
ing in three and one-quarter inch by three and one- 
quarter inch red and white oak wagon felloes, case- 
hardened in the dry kiln, compared with similar ones 
properly kiln dried. On the left is shown the same 
result produced by air drying in the case of willow 
oak. In Figure 25 the effect of final steaming in re- 
moving and reversing^ the casehardening is clearly 
shown in maple and oak. 

Warping. — As already explained, it is the internal 
stresses existing in the lumber which cause it to warp, 
twist, and cup. Casehardening is one basic cause of this. 



124 THE KILN DRYING OF LUMBER 

Another cause of warping is the direction of the 
grain of the wood, coupled with the fact that the shrink- 
age is different in different directions. Curly grain is 
very apt to bring this about. An " interlocking " grain, 
that is, one where the fibers of the successive layers 
twist spirally around the trunk in alternate directions, 
is even more effective. This gives a very tough wood, 
but one which is very much inclined to warp, and it 
does not hold its shape well even after it has once been 
dried. This interlocking grain is very common in many 
hard tropical woods such as lignum vitas, coca bola, 
and also in tupelo gum, and occasionally in red gum. 

It often happens that the trunks of trees twist 
spirally. The elements of the wood then are all in- 
clined to the axis in a spiral direction, irrespective of 
the fact that the annual rings may be uniform and con- 
centric. A board or stick sawed from such a log may 
appear as straight-grained because the intersections of 
the annual rings with the plane surfaces will be straight 
lines, but the wood will always split diagonally and 
the defect is designated as " spiral grain.'' This con- 
dition is illustrated in Figure 26, which represents such 
a log. If a board be sawed as indicated hj MN P,ii 
will intersect the annual rings in lines such q.^W XY Z 
parallel to its edges, but the grain in reality will be 
slanting and the board will split in a plane such as 
A B E F. This defect is frequently a great sur- 
prise because its nature is not understood — why 




B. 




m 

a> - 

p * 




tKSS!i«E«K«»*«H*«"«~~- 




HOW WOOD DRIES 125 

an apparently straight- 
grained piece of wood 
such as birch or maple 
should split diagon- 
ally. A board sawed 
from such a log will 
evidently tend to warp 
in drying. 

Shrinkage. — Un- 
equal shrinkage lies at 
the root of all troubles 
with handling wood. 
The relative amounts 
which different spe- 
cies shrink in the three 
directions vary con- 
siderably, but in gen- 
eral it may be stated 
that wood shrinks 
tangentially twice as 
much as radially, and 
one hundred times as 
much as longitudi- | 
nally. It is manifestly 
impossible, therefore, ^ 
for a piece of wood to 
dry below its fiber C 

saturation point with- Fig- Se.— Diagram explanatory of spiral grain. 




126 THE KILN DRYING OF LUMBER 

out producing internal strains. Were it not for its 
plasticity or colloidal nature, it would, therefore, 
always check in drying, but the wood is able to yield 
to these stresses and alter its shape accordingly. 
Imagine half a disk cut across a log forming a semi- 
circle with the medullary rays arranged as in an open 
fan. Draw a straight line parallel to its diameter 
across the face of the disk. When this shrinks 
the radii will close together slightly, just as in clos- 
ing the fan, and the straight line will curve out- 
wardly from the center. This is just the way 
a board will tend to curve in drying, and squares, 
circles, or any other figures cut from the log will dis- 
tort in drying in the same manner as though sketched 
upon the open fan and the fan then partly closed up. 
A quarter-sawed or radially-cut board, on the other 
hand, will remain flat, but will normally shrink more on 
the outer edges than at the center of the log. Figure 
27 shows how the shrinkage affects the shape. One 
view is diagrammatic and the other is the actual shrink- 
age of a disk of oak cut across the log. 

Not many data are available as to the longitudinal 
shrinkage of various species of wood, as ordinarily it 
is so slight that it may be neglected. For this reason, 
and on account of the very small thermal expansion, 
clock pendulums are frequently made of wood. Of our 
native species, redwood shrinks more longitudinally 
than the average, and mahogany is said to do the same. 



ffS' 



B<. 



It 





i>i a «gaa » wi « 8B w«MW g at^^ 




Fig. 28. — "Wash boarding" effect on a radially-sawed board of blue gum {Eucalyptus 

globulus). 




Fig. 29. — Remarkable shrinkage of California blue gum compared with redwood. All 
these boards were originally the same width. (The one on the right is redwood.) 



HOWi WOOD DRIES 127 

The shrinkage of wood in drying varies almost 
exactly in an inverse ratio with its moisture per cent. 
Thus, if plotted on cross section paper with moisture 
per cent, as an abscissa and width as an ordinate, the 
diagram forms a straight line from the fiber saturation 
point to zero per cent, moisture. 

The table (VII) on pages 129-131, which has been 
calculated from United States Forest Service tests,*^ 
gives the shrinkage from the green to the perfectly dry 
condition in the two directions across the grain and in 
volume, in per cent, of the green dimension. The vol- 
ume was measured on two inch by two inch pieces, and 
the radial and tangential on pieces two inches wide and 
four inches long in the direction measured. The 
weights per thousand board feet of lumber, exactly one 
inch thick when green, are also given for the green, 
shipping dry (25 per cent, moisture), air dry (15 per 
cent.), and kiln dry (3 per cent.) conditions. 

The pieces on which the shrinkage data were ob- 
tained were first air dried, and then placed in an oven 
heated to the boiling point until completely dry. 

In general the relative amount of shrinkage be- 
tween different species is somewhat proportional to 
density, the heavier woods shrinking the most, but there 
are some notable exceptions. Thus basswood, with a 
specific gravity of only 0.33, shrinks almost twice as 
much as black locust, with a specific gravity of 0.66. 

^arcular 213, Forest Service, U. S. Department of Agriculture. 



128 THE KILN DRYING OF LUMBER 

In a single piece of wood the reverse of this rule is 
often the case; in the case of blue gum (Eucalyptus 
globulus) the lighter wood of the concentric layers 
shrinks more than the dense heavy wood. Thus in dry- 
ing it corrugates and a radially-cut board (quarter- 
sawed) greatly resembles a washboard (Fig. 28). This 
remarkable kind of wood shrinks more than any other 
known species, and the shrinkage is not only different 
in the three principal directions, but varies from layer 
to layer. Consequently, it is the most difficult kind 
of wood to dry, especially from trees less than fifty 
years old. Moreover, as already noted, the shrinkage 
begins at about 80 to 90 per cent, of moisture, instead 
of at 25 to 30 per cent., as in most species. 

In some species, notably maple, oak, eucalyptus, 
and most of the conifers, the heartwood shrinks more 
than the sapwood, but in others, such as red gum, the 
reverse is the case. Where there is a difference in the 
amount of shrinkage between heartwood and sapwood, 
boards containing both are very apt to warp in drying. 
A quarter-sawed board, for this reason, will tend to 
bend sickle-shape. Thus radially-cut boards of red 
gum containing heartwood on one edge and sapwood 
on the other will bow outwardly in drying. 

The amount of shrinkage in a piece of wood is not 
a constant factor, but it varies with the manner in 
which the wood dries. Thus, when dried slowly at high 
temperature and high humidity, wood shrinks much 



HOW WOOD DRIES 



129 



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130 



THE KILN DRYING OF LUMBER 



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132 THE KILN DRYING OF LUMBER 

more than when dried very quickly in dry air. In fact, 
the very conditions which are the most suitable for 
drying the wood without checking are the most un- 
favorable from the shrinkage standpoint. 

In Figure 29 are several boards of Eucalyptus 
globulus and one plank of redwood, all of which were 
originally the same width, namely, nine inches. 

The shrinkages of these pieces from the green to 
the oven-dry condition were as follows, beginning with 
the specimen on the right-hand side: 

Per cent. 

No. 1. — Redwood, rings diagonal 4.8 

No. 6. — San Jose blue gum, from outer portion of board 3 feet wide 6.2 
No. 3. — Piedmont blue gum, radial, from outer portion of very 

wide board 12.3 

No. 9. — Piedmont blue gum, radial cut from inner portion of 

board 13.9 

No. 2. — Piedmont blue gum, rings diagonal to board 18.7 

No. 4. — San Josg blue gum, tangential cut 15.1 

No. 8. — Piedmont blue gum, tangential cut 23.2 

No. 10. — Piedmont blue gum, tangential cut (another tree) 23.2 

It is thus seen that the shrinkage in the tangential 
direction is nearly twice that in the radial direction, 
and that some trees of blue gum shrink very much more 
than other trees. One remarkable fact in regard to 
the shrinkage of this species is that, although the 
shrinkage is phenomenal in amount, the rate of shrink- 
age is no faster than that of redwood, but it starts 
to shrink much sooner, namely, at 80 or 90 per cent, 
moisture, whereas redwood does not shrink until it 
has reached 25 per cent, moisture. 



HOW WOOD DRIES 133 

The fact that the shrinkage is greater the slower 
the rate of drying, accentuates the conditions brought 
about in casehardening, since the outer shell, which 
dries the quickest, therefore tends to shrink the least, 
and the inner portion, drying much more slowly, 
ultimately shrinks the more, thus increasing the in- 
ternal stresses or the honeycombing. 

Shrinkage and "working of wood" can not be elim- 
inated entirely by any known means, although it may 
be reduced by impregnation with a solution of sugar, 
known as the '^Powellizing Process,"^ and it may be 
greatly retarded by a thorough coating of varnish or 
any moisture-resisting finish. In the Powell process, 
which is patented, the wood is impregnated with a 
solution of ordinary sugar, molasses, or beet sugar, 
about two to two and one-half pounds per gallon, by 
allowing it to soak in an open tank at 140° for an hour, 
then raised to the boiling point and boiled one hour 
for each one inch in thickness, then cooled in five to 
six hours. It is finally dried and then heated to about 
250° F. in order to caramelize the sugar. Sugar, which 
is a natural product of the cells, being transmutable 
with cellulose by the living processes, appears to enter 
Into intimate combination with the ultra-microscopic 
particles which compose the cell walls, in a manner 
which no other substance is able to do, and it greatly 
reduces the shrinkage, often to half or even less of 

* Powell Syndicate, Salisbury House, London Wall, London, E. C. 



134 THE KILN DRYING OF LUMBER 

the normal wood. A thorough coating of varnish will 
so greatly retard the absorption of moisture from the 
air or the loss in drying that immediate changes in 
weather will affect the wood very little. However, it 
does not remove the difficulty, it only postpones it, and 
ultimately the wood will shrink or swell, as the case 
may be, just as much as natural uncoated wood. The 
same may be said of impregnated wood or wood boiled 
in oil. Paraffin appears to be one of the most resistant 
coatings for wood in this regard. Experiment has 
shown it to be much more effective than linseed oil. 

Creosoting wood does not prevent its shrinking 
and swelling with changes in moisture, but merely 
retards the action. Wood which has been creosoted 
will swell just as much as untreated wood. Wood 
paving blocks which have been creosoted in a dry con- 
dition and then laid in the pavements will gradually 
swell, and if they have been tightly laid, they will 
inevitably cause the pavement to buckle up. They 
have been known to exert such a pressure in swelling 
as to shear off the curb stones. 

Only those substances which enter intimately into 
the cell walls affect the shrinkage. Creosote has been 
shown to do this to a small extent, and it produces of 
itself a slight swelling of the wood. 

Shrinking and swelling of wood will continue in- 
definitely with changes in the moisture content in the 



HOW WOOD DRIES 135 

air. Although the idea is prevalent that long season- 
ing reduces this tendency, there is no indication from 
tests that this is so. Neither do repeated soaking and 
drying reduce the swelling and shrinking. A white 
oak beam placed in the roof of *'01d South Middle" 
at Yale University in 1769 was removed in 1907 and 
tested for strength. A one-half -inch section from the 
middle of this beam was placed together with two 
similar pieces of white oak which had air seasoned 
about a year and left under a shed outdoors for seven 
months. Both samples were then tested for moisture. 
The beam contained 12.8 per cent, and the others 13.6 
per cent, and 14.1 per cent, respectively. They were 
then placed in a damp can over water with cover closed 
for fourteen days, when they had the following re- 
spective moisture contents : 26.6, 26.2, 26.4 per cent. 

In a cold climate in winter time the air in heated 
dwellings and buildings becomes excessively dry, even 
drier than would be considered suitable for a dry kiln. 
It ranges from 42 per cent, down to perhaps 15 or 20 
per cent. In the summer time with no heat on it may 
frequently rise to 90 or even 100 per cent, humidity, 
consequently the wood in buildings will continually 
swell in summer and shrink in winter. If well painted, 
the doors, floors, woodwork, etc., may continue to shrink 
in winter for three or four months after the fires are 
lighted, and will swell in summer as slowly. Thus 



136 THE KILN DRYING OF LUMBER 

these wooden parts will reach their greatest shrinkage 
in January or February and their greatest swelling 
in July or August. Observation for six years does 
not indicate any appreciable reduction in the amount 
of this "working'^ with age. 

There is, doubtless, however, a subtle advantage in 
slowly and long seasoned timber, and this consists in 
the probable gradual reduction in internal stresses, 
which are brought about by the drying. Wood must 
be looked upon as a plastic material, and it will gradu- 
ally give under continuous stresses. From this point 
of view long seasoned timber should possess certain 
advantages over quickly dried material, for certain 
exacting uses as for violins, drawing instruments, fine 
furniture, etc. 

Eepeatedly soaking and drying a piece of wood 
does not reduce its working, as has sometimes been 
supposed. In fact, it appears to slightly increase the 
shrinkage, due probably to the gradual leaching out 
of substances contained in the cell walls. Eecent tests 
made on a number of pieces of mahogany, which were 
alternately soaked and oven-dried seven times during 
forty-eight days, gave the following average results ; 

Total shrinkage first time 4.17 per cent. 

Total shrinkage seventh time 4.48 per cent. 

Green wood which has been once thoroughly oven- 
dried does not as a rule quite regain its original dimen- 
sions when resoaked. 



CHAPTER V 

The Principles of Kiln Deying 

The drying of lumber is an art wliich properly com- 
bines two distinct problems: One concerns simply the 
means for evaporating moisture, and the other has to 
do with the physical conditions involved in the ex- 
traction of the moisture from the wood with the least 
possible injury to the material. In this respect a dry 
kiln must be considered as something more than an 
apparatus for evaporating moisture. The second prob- 
lem has already been discussed in the previous chapter 
on *'How Wood Dries." The first problem will now 
be taken up in greater detail, with respect to its prac- 
tical operation or the art of drying. The theories in- 
volved in the operation will be treated of in a subse- 
quent chapter on the ''Theory of Drying." There 
are just three factors which the dry kiln engineer has 
at his disposal in controlling the drying of the lumber, 
namely, circulation, relative humidity, and temperature. 
Circulation is of the first importance because it is the 
sole means of conveying the heat to the lumber within 
a pile and for the removal of the evaporated vapor; 
without it drying on a commercial scale in the ordinary 
foims of kiln would be impossible. Where there are 
only a few sticks to be dried it is possible to heat these 

137 



138 THE KILN DRYING OF LUMBER 

by radiation or by direct contact with hot surfaces as 
in a veneer press, but when a pile of lumber is to be 
dried, the heat must be conveyed into the interior of the 
pile by a circulating medium, either air, steam, or other 
gas. So important is this fact that an entire chapter 
will be devoted to it. In the case of drying paper the 
evaporating of the moisture is brought about by direct 
contact of the sheet with hot iron rolls, or calenders 
as they are called, and circulation is not necessary. 
Also in drying veneer, this principle is made use of 
where the sheets are pressed between hollow iron plates 
heated by steam. (See Fig. 10.) In the case of lumber, 
however, these methods are ordinarily impracticable, 
even though possible, and it is therefore necessary that 
the air or heating medium circulate through every por- 
tion of the pile of lumber. Moreover, the other two 
factors are entirely dependent upon this one. If the 
circulation become sluggish at any point, there the tem- 
perature will fall and the relative humidity will rise 
and drying will cease or become very slow. 

The humidity is of next importance, because upon it 
depends the control of the drying operation and the 
proper condition of the lumber. By a high humidity 
the surface of the lumber is maintained in a moist 
condition while the water is being drawn from the 
center of the stick. Too low a humidity, either at high 
or low temperature, will generally result in caseharden- 



THE PRINCIPLES OF KILN DRYING 139 

ing, surface checking, and honeycombing. By the 
humidity in the air the surface is kept in a plastic and 
suitable condition for the transfusion of moisture 
through the wood substance. Furthermore, the rate of 
drying is completely controlled by the humidity. The 
best humidity will vary greatly with different species, 
moisture conditions and thickness. In the case of green 
oak and other dense hardwoods it should be held at 80 
or 90 per cent, until the wood has nearly reached its 
fiber saturation point, and then dropped gradually to 
about 40 per cent., whereas in the case of green birch 
or maple, drying may be started at 60 to 65 per cent, 
and ended at 15 or 20 per cent. Most softwoods may 
be dried at a relatively low humidity, but some, as 
western larch and cypress, require almost as high a 
humidity as oak. 

Heat is, of course, the factor which produces the 
drying. As in the steam engine, the steam and the 
mechanism are both essential, yet it is the heat which 
produces the result. It acts in two ways: the heat 
imparted to the water converts it into vapor, and the 
higher the temperature the greater is the capacity of 
the air, or more properly the space, for containing 
the vapor. Heat also appears to influence the rate of 
transfusion of the water through the wood substance. 
The higher its temperature the more rapidly it passes 
through. The temperature, moreover, has a marked 



140 ^THE KILN DRYING OF LUMBER 

influence on the physical character of the wood, which 
becomes more and more soft and plastic, while moist, 
as its temperature is raised. Strength is greatly re- 
duced with increase in temperature while moist. Here, 
too, the effect upon different species varies gTeatly. 
Some species, as Douglas fir and yellow pine, can be 
heated to the boiling point without injury and even 
may be dried at this temperature, while others, as 
oak, are injured by long exposure to high tempera- 
tures and can not be successfully dried under such 
conditions. A high temperature is not, as a rule, in- 
jurious to wood which is nearly dry, although long ex- 
posure will cause brittleness. On the other hand, boil- 
ing or steaming is not as a rule injurious, providing 
no drying is allowed to take place at the high tempera- 
ture. The danger occurs during the first part of the 
drying. It is very important that the wood be heated 
clear through to its center before any drying begins, 
as otherwise surface drying is apt to result, since it 
has been shown that water tends to move from the 
hot towards the cold portions of the wood. From the 
mechanical standpoint, high temperatures are more 
effective than low for evaporating moisture, but with 
wood it is the physical requirements of the substance 
which are the factors determining the temperature. 
The principles governing the drying operation may 
be summarized as follows: 



THE PRINCIPLES OF KILN DRYING 141 

The Process of Drying. — 1. The evaporating from 
the surface of a stick should not exceed the rate at 
which the moisture transfuses from the interior to 
the surface. 

2. Drying should proceed uniformly at all points, 
otherwise extra stresses are set up in the wood, causing 
warping. 

3. Heat should penetrate to the interior of the 
lumber before drying begins. 

4. The humidity should be suited to the condition 
of the wood at the start and reduced in the proper 
ratio as drying progresses. With wet or green wood 
it should usually be held uniform at a degree which 
will prevent the surface from drying below its fiber 
saturation point until all the free water has evaporated, 
then gradually reduced to remove the hygroscopic 
moisture. 

5. The temperature should be uniform and as high 
as the species under treatment will stand mthout ex- 
cessive shrinkage, collapse, or checking. 

6. Eate of drying should be controlled by the amount 
of humidity in the air and not by the rate of circula- 
tion, which should be made ample at all times. 

7. In drying the refractory hardwoods, such as oak, 
best results are obtained at a comparatively low tem- 
perature. In more easily dried hardwoods, such as 
maple, and some of the more difficult softwoods, as 



142 THE KILN DRYING OF LUMBER 

cypress, the process may be hastened by a higher tem- 
perature, but not above the boiling point. In many 
of the softwoods the rate of drying may be very greatly 
increased by heating above the boiling point with a 
large circulation of vapor at atmospheric pressure. 
In this case the dewpoint should be maintained at 
212° F. to prevent surface drying or casehardening. 

8. Unequal shrinkage between the exterior and in- 
terior portions of the boards and also unequal chem- 
ical changes must be guarded against by temperatures 
and humidities suited to the species in question to 
prevent subsequent cupping and warping. 

9. The degree of dryness attained should conform 
to the use to which the wood is put. 

10. Proper piling of the lumber and weighting to 
prevent warping are of great importance. 

How Lumber Should Be Piled. — In the next chapter 
the subject of relation of the shape of the pile to 
the circulation of the air will be discussed in some 
detail. It is the purpose here to point out how the 
size and shape of the pile influence both the time of 
drying and also the way in which the wood dries. 

In the first place it should be clearly understood 
that the interior of a pile of lumber can not be heated 
by radiation, but the heat must be carried into the 
pile by means of the currents of air passing through 
the layers of wood. This air will necessarily come in 



THE PRINCIPLES OF KILN DRYING 143 

contact with the outer layers first, to which it will give 
up some of its heat and through the evaporation it 
will take on more moisture ; consequently, the internal 
portions of the pile must necessarily lag behind the 
surface in the rate of drying. Herein is where many 
designs of dry kilns and drying methods have failed. 
To dry only a few pieces of wood is a totally different 
proposition from drying a pile of lumber. What can 
be accomplished in the case of a small test is often 
impossible when it comes to a large quantity, owing 
to the inability of obtaining the conditions in the inside 
of the pile, which are present in the small test. 

Furthermore, it should be clearly understood that 
the size of the pile of lumber which can be placed in a 
kiln is no criterion of the efficiency of the kiln for 
drying the lumber, in fact, it is more often true that 
the larger the pile, and the more tightly packed the 
kiln room, the less efficient is the apparatus. It is not 
the capacity of the room and the economy of space 
which should be considered primarily, but the prin- 
ciples upon which the drying depends. Eemember 
always that a dry kiln is a machine for drying lumber 
just as much as a veneer drier is a machine for drying 
the thin plates of wood, and it is not a storage ware- 
house where economy of space and handling are of 
the first importance. 

In considering the manner in which the air enters 



144 THE KILN DRYING OF LUMBER 

the pile of lumber, there are in general three cases 
for discussion. First, where the air enters and leaves 
irregularly at numerous points, a sort of diffusion proc- 
ess. This is what occurs in a loose, flat pile of air- 
dried lumber when placed in a. room with heating coils 
beneath and no specific form of circulation. This is 
the slowest system of drying and great variations in 
temperature and humidity are found to occur at dif- 
ferent portions of the pile, but on thei whole both 
sides of the pile dry about alike. It gives better re- 
sults with air-dried lumber than with green. Second, 
where the air enters the pile at one side and passes 
through in a definite direction, leaving at the opposite 
side. The direction of motion may be horizontal, as 
in flat, cross-piled lumber in a progressive ventilated 
kiln, inclined as in the Tiemann kiln, or vertical as in 
edge-piled lumber. In this case, which is the most 
definite and positive in its results, the drying is inevi- 
tably of a progressive nature. The portion of the pile 
where the air enters necessarily dries first and the 
portion where it leaves is the last to dry. The air 
enters the pile at its highest temperature and lowest 
humidity and leaves the pile at its lowest temperature 
and highest humidity and the conditions within the 
pile are progressive from the one to the other. The 
drop in temperature will vary with the rate of circu- 
lation, the width of the pile, and the moisture condition 



THE PRINCIPLES OF KILN DRYING 145 

of the wood. This case lends itself most readily to 
analysis. In fact, from a theoretical analysis of the 
operation such a pile exactly represents the conditions 
to which the Table IX, page 245, is directly applicable. 
Consider a slant pile in the kiln represented in Figure 
44, Chapter VIII. The air enters the heating coils at 
H at temperature Hn a saturated condition, is heated 
to ^2 s-^d humidity h^ in the flue and leaves the lumber 
at temperature t^ and a humidity of U^. With green 
lumber and a pile four feet in width with a velocity 
of circulation so that it requires only two to three 
seconds for the air to traverse the pile, the drop in 
temperature will be between 12° to 20° F. at the start 
with ^2 = 150° to 160°. As the lumber becomes dry the 
difference between t^ and t^ becomes less and less, being 
about 4° to 6° with t^ about 170° to 180°, but so long 
as drying is taking place there will always be a drop. 
In a direct comparison between two piles of inch 
green lumber, one of which was flat, so that the diffusion 
effect applied and the other inclined with direct circu- 
lation, the difference in temperature between the bot- 
tom and the center of the flat pile was 57° and in the 
inclined pile the greatest difference at the same time 
was only 14°. In the case of large ''diffusion" piles 
of green lumber the difference in temperature may in 
some cases easily exceed 100° during the beginning 
of the drying operation. 

10 



THE PRINCIPLES OF KILN DRYING 147 

The progressive manner in which the lumber dries 
in zones throughout the pile in direct circulation is 
well shown in Figure 30, in which are shown the drying 
curves for an inclined pile of wet red gum inch lumber, 
the width being four feet. The curve marked A is for 
a board on the entering side of the pile and that marked 
B for one on the side where the air left the pile. 

The horizontal distance between the two curves M-^i 
is, therefore, a factor of the width of the pile. It shows, 
furthermore, that while it would be possible to dry a 
few boards on a very narrow pile in the time shown 
by the curve A, the time required to dry any consider- 
able amount of material is increased to that shown 
by the curve B, since it is necessary to retain the entire 
pile in the kiln until the whole of it is dried to the 
required amount. 

From this it must be very clear that the length of 
time of drying is a function of the size of the pile in the 
direction of the air current. It is manifestly impos- 
sible, therefore, to prescribe a definite time of drying 
of any species or condition of lumber without taking 
into consideration the circulation and the size of the 
pile. The minimum time can be stated, however, for 
a single stick or for the curve A. 

Furthermore, since variations of 100° or more in 
temperature sometimes occur with the same pile of 
lumber, it is manifestly futile to specify the correct 



148 THE KILN DRYING OF LUMBER 

temperatures and humidities for drying lumber without 
a knowledge of the conditions of drying. Any such 
statement is misleading or even meaningless. 

In the case of the direct circulation system men- 
tioned above, the temperatures and humidities of the 
entering air must be the criterion, since to exceed the 
requisite conditions at this portion of the pile would 
cause damage to the lumber. It is not the average 
conditions which must control the operation, but rather 
the maximum conditions of the air which comes in 
contact with the entering side of the pile. 

In the drying curves given in Chapter XIII, page 
279, it is the temperature and humidity of the air enter- 
ing the pile of lumber which is there specified, for 
direct circulation. For other methods or forms of 
piling modifications must be made accordingly. 

The third case is where the circulation is reversible ; 
that is, where it first passes through in one direction 
and then changes and passes through in the opposite 
direction. Where this can be accomplished in a thor- 
ough manner the time of drying will be less than where 
it is always in one direction, and the two sides of the 
pile will dry more nearly alike. It is not easy of 
attainment, however, and arrangements for its accom- 
plishment are more apt to result in the slow diffusion 
process, with stagnation in portions of the pile. 

Brip and Condensation. — When drying at high 



THE PRINCIPLES OF KILN DRYING 149 

humidities, it often happens that the walls of the kiln, 
and particularly the ceiling, are below the dewpoint 
temperature of the air inside. Wherever this occurs 
condensation is certain to take place, popularly called 
' ' sweating, ' ' and will drip down upon the lumber from 
the roof or any overhanging parts, causing discolora- 
tion, and it may even prevent the drying when it occurs. 
This is a very disagreeable feature when it occurs on 
the ceiling. Hence it is of special importance to have 
the ceiling well insulated and moisture-proof, especially 
in northern climates. This condensing effect of the 
ceiling and adjoining portions of the walls may cool 
the air, in transverse circulating kilns, to such an extent 
as to greatly retard the drying. For example, sup- 
pose the drying temperature to be 160° F. at a humidity 
of 70 per cent. The dewpoint is then 146°, and unless 
the ceiling is warmer than 146° condensation will occur. 
One means of overcoming this trouble is to suspend 
a false ceiling about six inches below the ceiling proper. 
A better method, however, is to run several steam pipes 
along the ceiling sufficient to raise its temperature 
to slightly above 146° (in the example given) and the 
moisture at once disappears. Moreover, this will warm 
the upper currents of air and assist the drying. 

POEM OP THE DRYING CURVE 

The drying of a piece of wood under a uniform 
condition of the air does not occur at a uniform rate, 
but changes with the amount of moisture contained in 



150 



THE KILN DRYING OF LUMBER 




Fia. 31. — Air drying curves for two pieces of green western larch sapwood one 
inch thick, 3X3 inches square: A, with smooth surfaces; B, with rough surfaces. Note 
the change in rate of drying at the fiber saturation point, 23 per cent, moisture. 

the wood. In the case of green wood it is at first 
relatively rapid so long as there is any free water to 
evaporate, i.e., above the fiber saturation point. When 
the fiber saturation point has been reached, however, 



THE PRINCIPLES OF KILN DRYING 151 

there is a marked reduction in the rate, for the hygro- 
scopic water comes off much more slowly. A typical 
form of drying curve under uniform air conditions is 
shown in Figure 31, for two pieces of sap western 
larch. It will be noticed how abruptly the form of the 
curves changes as soon as the free water has evapo- 
rated, i.e., at the fiber saturation point. In order to 
accomplish the result in a reasonable time, therefore, it 
is necessary to increase the temperature or reduce the 
humidity, or both, when the fiber saturation point has 
been passed. It is important, however, to be sure 
that the free water has all evaporated from the center 
of the sticks, before the drying conditions are thus 
greatly augimented, as otherwise the evils of case- 
hardening will be encountered. 

The reason for this change in the rate at which wood 
dries is evident upon consideration. "When it first be- 
gins to dry the free water exists in all the pores and 
cell cavities all the way out to the surface, and the 
initial evaporation takes place just as from a free 
surface of water. As soon as the immediate wetness 
of the wet surface has disappeared further evapora- 
tion is hindered by the capillarity and the resistance to 
the passage of water through the wood. If drying 
takes place too fast at this point, the continuity of 
free water between the surface and the interior will 
be broken, and the rate of transfusion thus retarded. 



152 THE KILN DRYING OF LUMBER 

This is a condition to which the term ** casehardening " 
is often applied. A highly undesirable condition is 
thus brought about in which the interior of the stick 
still contains considerable free water, while the surface 
begins to lose its hygroscopic moisture. It is this in- 
ternal free water which is the most difficult to get rid 
of, and which is the chief factor wherein the drying of 
green wood differs from the drying of air-dried wood. 

When the free water has finally evaporated, the 
crucial point in the drying has passed, and there is 
little danger then of injury to the wood. The most 
critical point is when the surface has passed the fiber 
saturation point while the interior still contains some 
free water; that is, the stage where the greatest in- 
jury to the wood is apt to occur, unless conditions 
are exactly right. 

The hygroscopic moisture comes off much more 
slowly than the free water. As already explained, it 
can not be ''boiled" off. In fact, Dunlap has shown 
that to separate this hygroscopic moisture from the 
cell walls requires an appreciable expenditure of heat 
in addition to the latent heat necessary to vaporize it. 
This heat of adsorption amounts to about 34 B.t.u. per 
pound of dry wood at 32° F. 

Moisture transfuses through wood much more 
rapidly in the direction of the medullary rays ; that is, 
radially. Therefore, flat, sawed boards (tangential) 



THE PRINCIPLES OF KILN DRYING 153 

dry more rapidly than ' ' quarter-sawed, " ' ' edge-grain ' ' 
boards (radial). Furthermore, the condition of the 
surface affects the rate of drying, smoothed surfaces 
drying the faster. Figure 9 gives the drying curves 
at living room conditions of two carefully matched 
pieces of western larch sapwood, one with rough sawed 
surfaces, and the other planed. It is seen that the 
smooth surface dries slightly faster than the rough 
until the wood is nearly dry, when the reverse is the 
case. 



CHAPTER VI 

The Circulation and the Method of Piling ^ 

As already pointed out, in drying a separate in- 
dividual stickj the humidity and temperature may be 
regulated independently of the circulation. "When a 
large quantity of lumber is to be dried, so that it has 
to be placed in the kiln in the form of a pile, this is not 
the case. The temperatures and humidities within 
the pile become dependent to a large extent upon the 
circulation. A single stick may be heated by radiation 
from steam pipes or from the wall of the cylinder in 
which it is placed, but in a pile of lumber the heat must 
be conveyed to its inner portion by the convection or 
movement of the surrounding medium. This move- 
ment is essential, not only to supply the heat which 
is required to vaporize the moisture, but to remove 
the vapor as the air becomes saturated. This con- 
veyance of heat to the inside of the pile and removal 
of the excessive vapor may occur very slowly through 
a process of diffusion, as smoke will gradually diffuse 
itself through a room of perfectly still air, but the 
process is too slow to be of much assistance in the 
drying of a pile of lumber. 

* By permission from tlie Lumber World Review, article by the 
author. 

154 



THE CIRCULATION AND METHOD OF PILING 155 

The extreme slowness of this process of diffusion 
is evident if a little live steam be let into the top of a 
closed room of perfectly still air. The lower portion 
of the room will remain comparatively dry and free 
from steam, although there is no obstruction to the 
diffusion of the vapor. Direct conduction of heat is 
so small an element through the lumber and the air 
that it is not to be considered. In drying in water 
vapor alone in the absence of air, as with a vacuum in 
a closed cylinder or by superheated steam above 212° 
at pressures of one atmosphere or greater, the object 
of circulation is merely to supply the heat of vaporiza- 
tion, since the vapor will pass off of itself as rapidly 
as it is generated. If there were any way of getting 
heat to the sticks otherwise, circulation of the vapor 
would not be needed in the absence of air or other 
gas. For this reason a single stick can be very quickly 
dried in a vacuum when it can receive its heat by 
direct radiation, but the case is different in a large 
pile of material. For the same reason a board can 
be rapidly dried if placed between two hot plates of 
metal. 

Importance of Circulation. — From the foregoing 
statements it will be understood why the circulation 
of the air is of the first importance in the practical 
drying operations, for it is impossible to obtain correct 
humidities and temperatures within the pile unless there 



156 THE KILN DEYING OF LUMBER 

be ample, circulation in all portions. In fact, it may 
truthfully be said that circulation is the keynote to suc- 
cessful drying on a commercial scale. 

Natural Circulation is Downward. — ^In studying the 
natural air movement through a pile of moist lumber, 
it is found that the tendency is for it to descend and 
not to ascend. This action is caused by the fact that 
the evaporation taking place cools the air so that it 
becomes heavier.^ This increase in density, however, 
is not quite as self-evident as one might at first con- 
clude, because the additional vapor received from the 
wood has the opposite effect from the cooling and makes 
the mixture of air and vapor lighter. However, as 
shown mathematically, in Chapter X, the combined re- 
sult of the spontaneous cooling and increase in humidity 
increases the density sufficiently to cause the air to 
descend. 

This principle, that the air will spontaneously tend 
to descend in passing through a pile of moist lumber, 
is of great practical significance and plays a more im- 
portant part than is commonly supposed. 

Unless the piles of lumber in the dry kiln are so 
arranged that the above principle may operate, poor 
results will almost invariably follow, whether the cir- 
culation be produced by condensers, ventilation, or 
forced draught. 

» See Table X. 



THE CIRCULATION AND METHOD OF PILING 157 

Piles Constructed with Reference to the General Air 
Movement. — ^Another principle of almost equal impor- 
tance is that the lumber should be piled with reference 
to the direction of the air motion in the kiln. It should 
be so arranged that the path of least resistance lies in 
the direction of general air movement in the kiln, and 
that this path should lie in the spaces between adjacent 
layers of boards. That is, the stickers should run in 
the direction of the air current. The boards should 
not be placed so as to baffle the air currents. Other- 
wise the air will short circuit the piles and not pass 
through them. 

For example, in a progressive kiln in which the air 
enters at one end and passes out at the other, so that 
the general movement of the air is in a somewhat 
horizontal path through the kiln lengthwise, flat piling, 
crosswise, is a satisfactory method, provided that the 
boards are spaced sufficiently far apart to allow the air 
to sink downward at the same time that it moves through 
the pile. This condition is diagrammatically illustrated 
in Figure 32, where the ends of the boards are visible 
in the view, the stickers lying horizontally and length- 
wise of the kiln. 

Piling Methods Contrasted. — Where the air move- 
ment is in a general vertical direction, as is usually the 
case in compartment kilns, with condensers or with 
ventilators along the middle or the sides, flat piling 



158 



THE KILN DRYING OF LUMBER 




THE CIRCULATION AND METHOD OF PILING 159 

is not at all satisfactory, for the reason that the boards 
baffle the air currents. An actual test was made with 
two large kilns, in one of which the motion of the air 
was lengthmse from end to end, and in the other it was 
crosswise and vertical. Flat crosswise piling was used 
in both cases; and although the measured circulation 
was more than fifty times as great in the kiln with the 
vertical, transverse movement, the insides of the piles 
dried no better in the kiln with the enormously in- 




FiG. 33. — Cross section of kiln. Inclined piling is excellent where the circulation is 
chiefly in the vertical direction and transverse of the kiln. C, condensers; H, Heaters. 

creased circulation. It short-circuited the piles and 
did not pass through them satisfactorily on account 
of the baffling effect of the flat piled boards. 

Inclined piling, on the other hand, as indicated in 
Figure 33, or edge-piling, as in Figure 34, gives very 
satisfactory results with the vertical transverse method 
of circulation. It will be observed that in Figure 33, 
which represents a cross section of a condensing kiln. 



160 



THE KILN DRYING OF LUMBER 



the lumber is so arranged as to take advantage of the 
natural tendency of the air to descend in passing 
through the pile. This is in fact a better arrange- 
ment than the edge-piling shown in Figure 34, with 
heaters beneath the lumber and condensers on the sides, 
because the forced air movement is contrary to and is 
opposed by the natural tendency in Figure 34, and local 




Fig. 34. — Cross section of kiln. Edge piling is effective where circulation is vertical 
and transverse of the kiln. H, heaters; C, condensers. 

eddy currents are apt to occur, causing stagnation in 
places and irregular drying. 

The Best 'Arrangement. — The best arrangement of 
all is that shown in Figure 35 with edge-piled lumber, 
in which the condensers are placed beneath the pile 
and the heaters on the sides. This takes the fullest 
advantage of the principle of descending air and offers 
the least resistance to its motion. It will give much 
more uniform drying than the method illustrated in 
Figure 34, for there is no tendency here for the forma- 



THE CIRCULATION AND METHOD OF PILING 161 

tion of eddy currents or stagnation of the air at any 
point. Both factors are working in the same direction, 
instead of being opposed to each other as is the case 
in Figure 34. 

By way of caution to those who contemplate erect- 
ing new kilns, it should not be overlooked that there 
are many patents covering kilns which embody in 




Fig. 35. — Cross section of kiln. The best form of edge piling is where the downward 
principle is taken advantage of and condensers are below the lumber. 

various forms one or more of the methods here dis- 
cussed. The principles involved, as explained above, 
have not generally been clearly understood even where 
they have been utilized to greater or less extent in the 
patented kilns. 

Drying Should Take Place Equally/ from Opposite 
Surfaces. — A third principle of considerable impor- 
tance with certain kinds of sensitive woods is that the 
drying should take place equally from the opposite sur- 
faces of the boards or shaped blanks. If one surface 
11 



162 THE KILN DRYING OF LUMBER 

dries more rapidly than the other it will shrink less 
and a strong tendency to "cup" is produced, the curva- 
ture being in a direction away from the surface most 
rapidly dried. That is to say, the surface which dries 
most rapidly will be convex. The cause of this will 
be better understood by referring to the subject of 
casehardening, Chapter IV. This surface upon which 
the air impinges casehardens in a stretched condition, 
while the rest of the block is still wet. Then, as the 
rest of the block begins to dry very slowly, it shrinks 
more than this surface, thus causing the block to cup 
with the casehardened surface on the convex side. On 
account of this principle it is desirable in the case of 
sensitive woods that the air shall circulate through the 
pile of lumber in a direction parallel with the surfaces 
of the boards and not at right angles to the surfaces. 
This condition is fulfilled, in all four illustrations given, 
but is opposed when flat piling is used in transverse- 
vertical circulating kilns. 

If the lumber be dry, or no evaporation be taking 
place, there will be no downward tendency, unless the 
temperature of the surrounding air be warmer than 
that of the pile. It is, therefore, strongest when the 
lumber is moist and the air is dry. 

We will see presently that this observation is cor- 
rect theoretical reasoning; but before taking up this 
side of the subject there is another influence to be 
considered which generally comes into play. 



THE CIRCULATION AND METHOD OF PILING 163 

'A Cold Pile of Lumber Acts as a Condenser. — When 
a pile of lumber is first placed in the kiln, it is gen- 
erally considerably colder than the air in which it is 
placed. The result is that the pile of cold lumber acts 
as a condenser of enormous capacity and until it be- 
comes heated through to its normal temperature it 
will act very powerfully to draw the air downward 
through itself. This condition is very greatly aug- 
mented if the lumber is frozen when placed in the 
kiln. If anyone is in any way skeptical as to this 
action, let him crawl beneath a pile of frozen lumber 
soon after it is placed in the kiln. The downward 
draught of cold air will be suJBficient to remove all 
doubts whatsoever. 

This action takes place with little, if any, diminu- 
tion, even where the heating pipes are concentrated 
directly beneath the pile, and condensers are on the 
side of the kiln (see Fig. 41). Consider the refrigerat- 
ing capacity of a truck of green maple lumber holding 
approximately 5000 board feet. The dry wood will 
weigh about 2800 pounds per thousand, or 14,000 
pounds, and it will contain 60 per cent, moisture of 
the dry weight, or 8400 pounds of water, which is in 
the form of ice. Over four tons of ice, therefore, must 
be melted before the lumber can even begin to warm 
up, or any drying take place. This will require an 
expenditure of 1,201,200 British thermal units just to 



164 THE KILN DRYING OF LUMBER 

melt the ice alone, without any heating effect. If this 
must be heated from, say, 32° to 102°, an additional 
expenditure of 70° times 8400 pounds plus 70° times 
the specific heat of the dry wood (70 X 8400) plus 70° 
(14,000 X 0.33) equals 911,400 British thermal units, 
or a total expenditure of 2,121,000 British thermal units 
is required before drying begins. 

To raise the temperature of one cubic foot of a 
mixture of saturated vapor and air at 102° and 
atmospheric pressure one degree, requires 0.01686 
B.t.u.,^ assuming this air and vapor to enter the pile 
at, say, 122° at 57 per cent, humidity and to cool until 
it becomes saturated, which will be at 102° (no evap- 
oration taking place, the increase in humidity being 
due solely to the cooling from 122° to 102°), it 
will require a minimum of 2,121,000-^ (0.01686 X 20°) 
equals 6,290,036 cubic feet of air (measured at 102°) 
or 6,513,881 cubic feet (measured at 122°) merely to 
warm the load of 5000 board feet to 102° without pro- 
ducing any evaporation, assuming all of its heating 
capacity to be utilized to the maximum amount in pass- 
ing through the lumber. 

As a matter of fact, it would probably require double 
this amount, since some of it would pass through with- 
out parting with all of its heat. This volume of air 
would occupy a room 20 feet wide by 20 feet high 

» See Chapter X. 



THE CIRCULATION AND METHOD OF PILING 165 

and 15,725 feet, or approximately three miles, long. 
This is the minimmn quantity of air which must come 
in intimate contact with the lumber under the assumed 
conditions to warm the pile up to 102° from a frozen 
condition at 32° before any evaporation takes place. 

From this simple illustration some conception may 
be had of the magnitude of this factor of downward 
circulation, produced by the coolness of the lumber alone 
when first placed in the kiln before evaporation begins. 

Variations in temperature of the air in the kiln 
will, of course, influence the motion of the air through 
the pile. If the kiln is cooling, the condition may be 
temporarily reversed, since the lumber may then be 
warmer than the air. "When the temperature in the 
kiln is rising, on the other hand, the conditions may 
be accentuated. 

Temperature of Wet Lumber is Same as Wet Bulb 
of a Hygrometer. — ^It has been observed in a previous 
chapter that the temperature of a wet piece of lumber 
will be less than that of the surrounding air, except 
when the air is saturated. While the surface is wet 
it will, in fact, correspond to the temperature of the 
wet bulb of a hygrometer. This cooling is due, of 
course, to the evaporation taking place. When the 
surrounding air becomes saturated evaporation com- 
pletely ceases, and the temperature of the wood will 
gradually rise to that of the saturated air. As the 



166 THE KILN DRYING OF LUMBER 

surface of the wood becomes drier and drier its tem- 
perature will gradually rise from that of the wet bulb 
to that of the dry bulb. When the wood is dry to the 
point where it is in equilibrium with the relative 
humidity of the surrounding air, evaporation ceases 
and its temperature will then become that of the air. 

The following experiment illustrates excellently the 
action explained above which takes place when a piece 
of frozen, wet wood is first placed in a dry kiln. A 
stick consisting of a piece of black walnut two and a 
half inches thick, containing about 50 per cent, moist- 
ure, which was frozen clear through, was placed in the 
dry kiln alongside of a large pile of the same material 
in position where it received free circulation on all 
sides. A hole was bored in the center and a glass 
thermometer inserted and carefully plugged and 
insulated. 

The weights in grammes, temperatures and moist- 
ure per cents are given in the table. 

Notice that the stick actually increased in weight 
for the first two and one-quarter hours, and that con- 
siderable water condensed upon it in addition and 
dripped off, which was not included in the weight. Dry- 
ing did not begin until the stick had been in the kiln 
for four and one-half hours, and after eight and a 
quarter hours it was still considerably wetter than 
when first put in. Comparing the temperature inside 



THE CIRCULATION AND METHOD OF PILING 167 

Table VIII. — Comparison of Temperatures Inside and Outside a 
Stick of Wet Wood Placed in a Dry Kiln. 



Number 
of hours 
in kiln 


Weight in 
grams 


Tempera- 
tures in- 
side of 
stick, F". 


Wet 
bulb 

F." 


Dry 
bulb 

F.° 


Hygrom- 
eter rela- 
tive 
humidity 
per cent. 


Remarks 
Frozen 





4004 


28 








Considerable water 


M 


4040 


44 


87 


93 


78 


dripped oflf in ad- 


IM 


4045 


60 


87 


93 


78 


dition to that 


IM 


4048 


67 


87 


96 


70 


weighed. 


21^ 


4050 


73 


87 


96 


70 




2H 


4050 


75 


87 


96 


70 




3M 


4050 


80 


87 


96 


70 




3M 


4050 


82 


87 


96 


70 




4M 


4047 


84 


87 


96 


70 


Drying begins. 


m 


4045 


84 


87 


95 


72 




5H 


4042 


84 


87 


95 


72 




5H 


4042 


84 


87 


95 


72 




6Ji 


4040 


85 


88 


96 


72 




6M 


4038 


86 


88 


96 


72 




7H 


4036 


87 


88 


96 


72 




7H 


4035 


87 


88 


96 


72 




8M 


4031 


87 


87 


95 


72 




23M 


3985 


90 


89 


97 


73 




25 


3978 


89 


89 


97 


73 




30 


3948 


88 


87 


95 


72 




45 


3875 


97 


94 


104 


69 




50 


3855 


97 


94 


106 


63 




144 


3542 


97 


92 


103 


65 





the stick with that of the wet bulb of the hygrometer, 
it is interesting to see that it required eight and a 
quarter hours for it to reach the temperature of the 
wet bulb and that after twenty-five hours they still 
corresponded. 

Precautions in Observing and Determining the Cir- 
culation. — The movement of the currents of air is some- 
times likened to streams of water in free air. Such a 
conception is liable to lead to false reasoning and erro- 



168 THE KILN DRYING OF LUMBER 

neous conclusions. A stream of air does not flow in 
the same sense as a stream of free water in the air 
does, but it rather floats within itself, being balanced 
by the surrounding portions, and its motion is brought 
about by differences in pressure and momentum, in- 
stead of directly by gravity, as in falling water. 

Differences in pressure may be caused by differ- 
ences in density, or by mechanical movement of the 
adjacent portions of air or solid bodies. Until one 
gets rid of the conception of flowing water it is fre- 
quently a surprise to discover the manner in which 
the air is moving. Hot air does not necessarily ascend 
unless there is a column of denser cold air to displace 
it. The actual movement of air through a pile of 
lumber is sometimes a difficult matter to determine. 
It is frequently too slow to be measured by an anemom- 
eter. Smoke suggests itself, but unless the smoke is of 
the same temperature as the air, it will give erroneous 
indications and it must not be given an initial velocity 
as when blown from the mouth. 

I have found that the use of Chinese punk or in- 
cense sticks, such as are sold by all druggists for keep- 
ing away mosquitoes, is the most satisfactory method. 
Several of these sticks may be inserted within the 
pile of lumber, and the movement of the fine threads 
of smoke observed by an electric hand light. Even 
in these small sticks, however, there is sufficient heat 



THE CIRCULATION AND METHOD OF PILING 169 

at the glowing points to produce a decided initial up- 
ward velocity to the smoke, which must always be 
taken into consideration in making the observations. 

Temperature Throughout the Pile an Indication of 
Circulation. — Another way of determining whether the 
circulation is sufficient through the pile is by the tem- 
perature. If it is sluggish at any point, there the 
temperature will be considerably lower than normal. 
In order to determine this, thermometers with stems 
several feet long are necessary, so that the bulbs can 
be inserted into the interior. Thermometers with long, 
flexible tube connections between the bulbs and the in- 
dicators are suitable. 

To show what this temperature variation may 
amount to, it will suffice to describe conditions in an 
experiment in drying inch green maple, already re- 
ferred to in the last chapter. In this case two piles 
were placed together in a smaU compartment kiln, one 
of which was flat piled and the other inclined, as shown 
in Figure 33. A curtain was dropped between the 
two to prevent mixing the air currents and in all 
respects they were subjected to the same conditions. 
In the flat pile, however, the boards baffled the circu- 
lation and it was sluggish, whereas in the inclined pile 
it was excellent through all portions. After being in 
the kiln for nine days, the temperatures in various 
locations were as follows : 



170 THE KILN DRYING OF LUMBER 

Plat Fiie 

Beneath pile 184 degrees 

At center of pile 127 degrees 

Above pile 158 degrees 

Variations between bottom and center ... 57 degrees 

Incmned Pile 

Beneath pile 172 degrees 

At center of pile 164 degrees 

Above pile 158 degrees 

Variations between bottom and center ... 8 degrees 

The inclined pile dried thoroughly and uniformly, 
whereas the flat pile was covered with mold at the 
center and not thoroughly dry when both were unloaded 
at the same time. 

Diagrams Showing Circulation Actually Observed 
in Various Piles. — In order to more fully establish the 
truth of the principles which have been discussed, a 
few diagrams will be given showing the circulation as 
actually observed. 

In these diagrams the full arrows indicate the 
observed circulation and the dotted arrows the prob- 
able. The relative rates of the air movements are 
indicated approximately by the lengths of the arrows. 

Figure 36 shows one of three piles of green maple 
inch lumber of about one thousand feet each, boards 
edge-stacked, with the stickers running vertically. The 
heating pipes were below the pile and a condenser 
chamber on either side of kiln. The top of the par- 



THE CIRCULATION AND METHOD OF PILING 171 



tition separating condenser chamber from kiln is shown 
above the lumber. Notice that in spite of the free 
open vertical passages the air was descending in some 
places. 



^ 



I 
J 



^M hi, of. jv^j^HUnj tt CtyuJenainj Cht 

I f.Ji.{AAf^it.t.t,T,l.r.t.] 




Fia. 36. — Edge-piled lumber. Longitudinal section through kiln. Actual case — 
note that in spite of the fact that circulation vertically is entirely unrestricted, with 
heating pipes directly below the pile of lumber and suction above over into the condens- 
ing chamber, nevertheless the circulation was downward through the left-hand side of 
the pile. 

Figure 37 shows an inclined pile with the incline 
in the wrong direction. This was also a pile of green 
one-inch maple, eight feet high and five feet wide on 
top and eight feet long. This occupied the same rela- 



172 



THE KILN DRYING OF LUMBER 



tive position in the kiln as did that in Figure 36. Notice 
here that the air is moving in the wrong direction near 
the top of the pile on one side. The lumber did not dry 
thoroughly in this position. 



v^///////////////////////;^/////////////////////A 




f Soi&omotf Turttiion t 



^\\\\\\\\\\\\\\\\\\\^^^^ 



Fig. 37. — Inclined pile of boards, sloped in the wrong direction. Actual case— 
this is in the same position in the kiln as the edge pile Figure 1. Note that the air is 
moving in the wrong direction in the upper left-hand portion of the pile. 



Figure 38 shows an inclined pile of green inch maple 
boards which is sloped in the proper direction to take 
advantage of the descending tendency of the air. This 
is placed lengthwise of the kiln, a condensing chamber 
on either side. The circulation here was correct 



THE CIRCULATION AND METHOD OF PILING 173 

through all portions and the lumber dried uniformly. 
It is possible to so arrange the condensers that 
they oppose the natural draught and actually retard 
the circulation, causing stagnation. Such a case is 
illustrated in Figure 39, in which drying practically 
ceased, although the humidity above the pile was less 
than 20 per cent. 




Fig. 38." 



-Inclined pile of boards correctly sloped. Cross section of kiln. Actual case. 
Note the excellent circulation. 



Example of Downward Circulation in Ventilated 
Kiln. — ^A remarkable example of the downward circula- 
tion through a pile of lumber in a ventilated kiln of a 
box form, as indicated by the temperature, is shown 
in Figure 40. This was a simple wooden kiln about 
seven feet wide, fourteen feet high, and seventy feet 
long. A single layer of steam pipes in the bottom 
on either side of an intake air duct and several stove- 



174 



THE KILN DRYING OF LUMBER 



pipe ventilators were placed along the middle of the 
roof. 

The Mln was loaded with green sticks one and one- 
half inches square by twenty inches long, piled criss- 
cross, with about one-fourth inch space between. The 
pile was raised above the rails on 2 X 8 skids, as 
shown. 

Eecording thermometers with long flexible tube con- 






^B,^ 



\i 







I 



Fig. 39. — Caae in which the condenser is so placed as to ojjpose the natural circula- 
tion and cause stagnation in part of the pile. Conditions were improved in this case by 
entirely closing ofE the condensing chamber by boarding over the top of the partition 
just beneath the condenser. 

nection to the bulbs were placed in the pile and near 
roof and sides of kiln as indicated. The lower ther- 
mometer A was shielded from direct radiation from 
the steam pipes by a short piece of inch board, the 
same length as the bulb, placed beneath it. The read- 
ings of these thermometers at periodic intervals are 
given in the following table ; 



THE CIRCULATION AND METHOD OF PILING 175 
Reference to Fig. 40 

Days Thermometer 

in Readings F,° 

kiln ABODE 

1 60 114 120 119 117 Ventilators closed 

2 106 130 133 131 127 Very little steam on in pipes 

3 107 113 123 131 114 

4 130 109 114 124 111 Roof ventilators open 
8 113 99 122 129 106 Full steam on 

10 110 98 123 131 107 
12 110 99 131 134 112 










yjZ^/////.r/my(j. 



^m. 



Fig. 40. — Downward circulation in a ventilated kiln observed by temperature 
measurements and moisture determinations. (See tables.) Note that this kiln was 
manifestly designed with_the idea that the air would pass upwardly through the lumber. 



176 THE KILN DRYING OF LUMBER 

Moisture tests after sixteen days in kiln were as 
follows : 





Bottom 


Middle 


Top of 
pile 


1 


44.7 


38.6 


11.5 per cent, of dry weight. 


2 


52.4 


43.7 


13.2 per cent, of dry weight. 



It will be seen that in spite of the arrangement being 
such as to induce an upward movement from the pipes 
in the bottom to the ventilators in the roof, the pro- 
gressive drop of temperature from D to C to B shows 
conclusively a downward movement of air through 
the pile. Thermometer A beneath the pile was no 
doubt affected by radiation from the steam pipes. 

Forced Draught. — In the case of movement of air 
by fans or blowers through a pile of lumber, the motion 
at any point is produced by differential in pressures 
at that point. Ordinarily the momentum of the air 
can not be counted upon, since it is broken up by passage 
through the lumber. Between any two portions of the 
kiln which differ in pressure the air will flow along the 
path of least resistance. Since the resistance of various 
portions of the pile must necessarily differ greatly, the 
velocities of the air will also differ accordingly. In 
this respect forced draught differs essentially from 
natural circulation produced by differences in density. 
The self-regulatory feature is also lacking in forced 
draught. As has been pointed out in the natural cir- 



THE CIRCULATION AND METHOD OF PILING 177 

dilation, the colder or wetter portions of the pile will 
automatically draw the greater portion of the air 
towards them, which is a highly desirable feature. 

There is yet another condition which is unfavor- 
able with forced draught. Unless the draught is in- 
variably in the same direction as the natural tendency, 
it will operate against it ; and if the two opposing forces 
happen to balance each other at any point, stagnation 
must occur and drying is not only not accelerated, but 
is actually retarded or even prevented at such points 
by the forced draught. For these reasons forced 
draught of itself is not apt to give uniform drying, 
except when it is properly combined with the natural 
system. If this arrangement can be brought about, 
however, forced draught may be made of assistance. 

Progression of Drying Through the Pile. — ^When- 
ever the circulation has a constant direction through a 
pile of lumber, it follows that the surface where the 
air enters will dry in advance, and that where the air 
current leaves, the pile will dry last (Fig. 30). The 
temperatures and humidities will also be graduated 
through the pile accordingly, the highest temperature 
a-nd the lowest humidity being that of the entering air. 
Suppose the air enters the pile at a temperature of 
158° and 13 per cent, humidity and leaves at 75 per 
cent, humidity. Calculation shows that the air will 
have cooled spontaneously to 107°, due to the evapora- 

12 



178 



THE KILN DRYING OF LUMBER 



tion which, occurs. It is evident that the rate of evap- 
oration will be most rapid at the point of incidence and 
will gradually become less towards the point of exit. 
As the lumber nearest the impinging air dries, the 
evaporation rate will increase progressively through 
the pile until the most remote portion finally becomes 
dry. The progress of drying a pile of lumber is there- 




Fia. 41. — Circulation as observed in a pile of cold lumber shortly after being placed 
in the kiln — note the downward movement in spite of the fact that the heating pipes are 
directly beneath the pile and condensers are in operation on the two sides. Plus sign (+) 
indicates motion toward observer, and minus sign ( — ) away from observer. 

fore a progressive one throughout the pile, occurring 
in successive zones. Therefore, it is fair to conclude 
that the size of the pile in the direction of the air 
movement determines the length of time necessary to 
dry a given lot of lumber, since the load can not be 
considered ready to be removed from the kiln until 
the most remote pieces have arrived at the desired 
degree of dryness. 



1 



THE CIRCULATION AND METHOD OF PILING 179 

While conditions in practice will not always agree 
in detail with the typical case explained above, since 
there are more often vacillating currents through the 
pile, the principle must be taken into account in figuring 
upon the length of time required to dry under given 
conditions. For this reason it is obviously erroneous 
to suppose the time of drying in practical operations 
for a given kind of wood to be a definite quantity, 
since the form and size of the pile determine the time 
as well as does the kind of wood. 



CHAPTER VII 

Special Peoblems in Buying 

Kiln Drying. — ^Were it not for the unequal shrink- 
age and the slow rate of transfusion of the moisture 
from cell to cell, the drying of lumber would present 
no more difficulty than the drying of wet cloth or clay. 
The problem would be merely one of conducting the 
requisite amount of heat to the material to supply that 
required for vaporization, which at 163° F. is 1000 
British thermal units of latent heat plus a small addi- 
tional amount (about 34 B.t.u. per pound of dry wood)^ 
required to overcome the attraction of the hygroscopic 
material for the moisture. By use of a low pressure 
or a temperature higher than the boiling point the 
moisture would pass off directly in proportion to the 
quantity of heat supplied. 

With few exceptions, however, this condition of 
affairs does not apply in the case of lumber except in 
the form of thin veneer. The reason for this differ- 
ence will be made clear in the present chapter. 

Properties of the Wood Which Affect Drying. — 
In the first place let us review the physical properties 
of the material, which must be recognized in order to 
intelligently study the drying problem. Different 

* Frederick IVunlap. 
180 



SPECIAL PROBLEMS IN DRYING 181 

species differ very greatly with respect to the relative 
proportions of these properties, but all possess them 
more or less. 

1. The rate of transfusion of moisture through the 
wood substance has already been discussed. It is very 
slow in some woods, as oak, and fairly rapid in others, 
as pine. It is supposed that the rate is accelerated by 
increase in temperature. 

2. Wood shrinks differently in different directions, 
and in different portions of the same piece. Shrinkage 
usually begins only when the dryness falls below the 
fiber saturation point, although with some species, as 
Eucalyptus globulus and some oaks, the point is not 
well defined. It is greatest in the circumferential direc- 
tion of the tree, being generally twice as great in this 
direction as in the radial direction. In the longitudinal 
direction, for most woods, it is almost negligible, being 
from twenty to over a hundred times as great circum- 
ferentially as longitudinally. There is a great varia- 
tion in different species in this respect. Consequently, 
it follows from necessity that large internal strains are 
set up when the wood shrinks, and were it not for its 
plasticity it would rupture. In some species, such as 
Eucalyptus globulus, the alternate layers of early and 
late wood of the annual rings shrink very differently 
in amount, thus causing additional strains and stresses. 
There is an enormous difference in the total amount 



182 THE KILN DRYING OF LUMBER 

of shrinkage of different species of wood, varying from 
a shrinkage of only 7 per cent, in volume, based on the 
green dimensions, in the case of some of the cedars to 
nearly 50 per cent, in the case of some specimens of 
Eucalyptus globulus and Eucalyptus viminalis. 

3. Wood substance becomes soft and plastic at high 
temperature under moist conditions. The effect of 
temperature upon plasticity varies greatly with differ- 
ent species, some, as western red cedar, redwood and 
eucalyptus, becoming excessively soft even as low as 
150° or 170° F. 

4. Cohesion between the fibers easily breaks down 
with increase in temperature in such woods as western 
larch and the southern swamp oaks, thus permitting 
internal stresses to cause checking with great readiness. 

5. Tendency to warp is due to a warped direction 
of the fibers. Cupping of slab cut boards is simply 
explained by geometrical relations due to unequal 
s.hrinkage radially and circumferentially. 

6. Wood shrinks more when dried slowly under 
moist conditions than when dried rapidly. High tem- 
peratures under moist conditions are conducive to 
greater shrinkage. 

7. Excessive drying causes brittleness. 

8. Wood absorbs or loses moisture in proportion to 
the relative humidity of the air, irrespective of tem- 
perature. This property is known as ' ' hygroscopicity. ' * 



SPECIAL PROBLEMS IN DRYING 183 

9. Hygroscopicity and *' working" are reduced but 
not eliminated by thorough drying. 

10. Moisture tends to transfuse from the hot 
towards the cold portion of the wood. 

11. Change of color occurs in some species in dry- 
ing. This is distinct from sap stain or colors caused 
by fungus or bacteria. This is notable in hard maple 
sapwood and in sugar pine. In the maple, a moist, 
warm atmosphere is conducive to this coloration. 

12. Collapse of the cells may occur in some species 
while the wood is hot and moist. This collapse is dis- 
tinct from the shrinkage which takes place in the wood 
substance and is due to a different cause. 

13. When the free water in the capillary spaces of 
the wood fiber is evaporated it follows the laws of 
evaporation from capillary spaces, except that the pas- 
sages are not all free passages, and much of the water 
has to pass out by a process of transfusion through 
the moist cell walls. These cell walls in the green 
wood completely surround the cell cavities so that 
there are no openings large enough to offer a passage 
to water or air. The well-known "pits" in the cell 
walls extend through the secondary thickening only 
and not through the primary walls. This statement 
applies to the tracheids and parenchyma cells in the 
conifers (gymnosperms) and to the tracheids, paren- 
chyma cells, and wood fibers in the broad-leaved trees 



184 THE KILN DRYING OF LUMBER 

(angiosperms) ; the vessels in the latter, however, form 
open passages except when clogged by ingrowths called 
tyloses, and the resin canals in the former sometimes 
form occasional openings. By heating the wood above 
the boiling point, corresponding to the external pres- 
sure, the free water passes through the cell walls more 
readily. 

To remove the moisture from the wood substance 
requires heat in addition to the latent heat of evap- 
oration because the molecules of moisture are so in- 
timately associated with the molecules or minute par- 
ticles composing the wood that energy is required to 
separate them therefrom. Carefully conducted experi- 
ments by Mr. Dunlap show this to be from 16.6 to 19.6 
calories per gramme of dry wood in the case of beech, 
longleaf pine and sugar maple. The difficulty imposed 
in drying, however, is not so much the additional heat 
required as it is in the rate at which the water trans- 
fuses through the solid wood. 

Causes of Various Effects Which Result from Dry- 
Ing^ — The foregoing facts will account for nearly all 
the phenomena and troubles which occur in kiln drying. 

ChecMng, warping, honeycombing, are due to un- 
equal shrinkage, crooked grain and casehardening (see 
Fig. 8) . ' ' Washh oar ding ' ' is due to unequal shrinkage of 
adjacent layers of the annual rings of wood and appears 
on radially sawed lumber (quarter sawed), Fig. 28. 



HZ 



•-J m 

Sg 

(0 CO 
QQ a 



CLco 

O O^ 
O Q 





Fig. 43. — Magnified sections of western red cedar boards, showing the beginning of the 
"Explosive" effect. 



SPECIAL PROBLEMS IN DRYING 185 

CaseTiardening has been discussed in the previous 
chapter. 

Discoloring.— It has been found that certain species, 
as hard maple, change to a cherry color when exposed 
to very moist conditions at rather high temperature. 
The very conditions which are best for elimination 
of checking and casehardening are especially conducive 
to this coloring. Fortunately, maple, however, does 
not check easily even in a dry atmosphere, and a tem- 
perature of 120° to 125° with a humidity of 50 to 70 
per cent, may be used with the green wood, thus avoid- 
ing the discoloration. The color appears to be due to 
an oxidation of a constituent in the sapwood, since it 
does not occur in the total absence of oxygen. The 
brown stain, so serious in the case of western sugar 
pine, may be due to a similar cause. 

Collapse of the cells, mentioned under ''Properties 
of the Wood," occurs notably in western red cedar, 
when the very wet wood is dried at a high temperature. 
It usually occurs in streaks in the sapwood of the butt 
log, which is full of water. Fig. 42 shows this col- 
lapsed condition in the two boards to the right, and 
how it was completely overcome by drying at a lower 
temperature in the similar two boards on the left. 
This is entirely distinct from the phenomena of shrink- 
age, and is accounted for by a theory which appears 
to be borne out by experiments. This is, that the cells 



186 THE KILN DRYING OF LUMBER 

being inclosed capillary spaces and full of water, as the 
water soaks out through, the walls no air can enter 
and the water exerts a tensile stress sufficient to draw 
the walls together if they are sufficiently plastic to 
yield to the stress. It is a well-established fact that 
water contained in small spaces can exert a tensile 
stress of cohesion equal to many atmospheres. The 
solution for the red cedar is to keep the temperature 
below 135° or 140° so long as the walls are wet. Eed- 
wood exhibits a similar phenomenon, and it is prob- 
able that it occurs to a modified degree in nearly all 
woods. 

Explosion. — ^By heating the wet western red cedar 
above the boiling point, the opposite condition from 
collapse is brought about — the vapor pressure gener- 
ated within the non-porous cells forces the walls out- 
wards, and causes the wood to bulge on the surface. 
The internal condition of an '' explosion," as it may 
be termed, is shown in the section Fig. 43. 

Mold. — ^Mold on the surface of lumber will grow 
vigorously in a moist or saturated atmosphere at 110° 
to 130° F. Unfortunately what is the best condition 
for drying the majority of green hardwoods is the 
optimum condition for its development. Its growth is 
checked, however, at 140° F., and 150° to 160° will kill 
it or prevent further development. This temperature 
does not sterilize the wood, or render it immune, nor 



SPECIAL PROBLEMS IN DRYING 187 

even 212°, since mold will develop on wood which has 
been steamed. The main objection to the formation of 
mold is that it grows across the openings between the 
layers of lumber and mechanically retards or checks 
the circulation. It also looks bad. It does not, how- 
ever, penetrate the lumber or injure it in any way. 
Dry air will prevent the mold, but is unsuitable for 
drying green lumber. A temperature above 140°, as 
stated above, will also prevent it, if the heat can be 
obtained clear through the pile, but is likewise preju- 
dicial to many kinds of lumber when dried from the 
green condition. While mold is very often an indica- 
tion of stagnation of air conditions where it occurs, 
it is not by any means necessarily so, since it will 
develop in a high circulation of air if the air is nearly 
saturated and is at a temperature of 110° to 130°. 

A simple means of temporarily alleviating the diffi- 
culty, if mold begins to develop seriously, is to steam 
the lumber at 160° to 180° for half an hour or an hour 
by injecting live steam into the air through perforated 
steam pipe suitably placed so as to force the circula- 
tion through the pile. This will kill the mold already 
formed, and it is not likely to develop again. There 
need be no fear of injuring the lumber by such treat- 
ment, at any stage of the drying. 

Too long drying at high temperatures even below 
212° renders wood brittle and weakens it. Eepeated 



188 THE KILN DRYING OF LUMBER 

drying even at 212° gradually changes wood through 
slow distillation into charcoal. Where the strength of 
the wood is important, high temperatures must be 
avoided. In such cases it is unwise to dry the wood 
much beyond the condition in which it is to be used. 
Thus, for wagon stock, a dryness of 8 per cent, should 
be sufficient, if it is uniform, and the material should 
be allowed to again take up 2 or 3 per cent, moisture 
in the shed before it is put into the wagon. 

The effect of drying on the strength is given in 
Chapter XI. 

Coloring Wood hy High Temperatures. — Steaming 
is sometimes resorted to to darken the color of wood, 
especially the sapwood, as in the case of black walnut 
and red gum. The higher the temperature and the 
longer the exposure the greater is the effect. Steaming 
at atmospheric pressure for twenty-four to forty-eight 
hours will considerably darken the color. Steaming 
under pressure at five pounds for two hours will accom- 
plish excellent results with black walnut, or one and a 
quarter hours with mahogany. Severe treatment in 
saturate steam, however, is decidedly detrimental to 
the strength of the wood, as shown in Chapter XI. 

Recent experiments carried on at the Forest 
Products Laboratory have shown that by treating the 
perfectly dry wood in dry air much higher tempera- 
tures can be used with, less injury. Eed gum is ren- 



SPECIAL PROBLEMS IN DRYING 189 

dered tlie color of rich dark rosewood by subjecting it 
to 400° F. for four to six hours. Yellow birch, white 
ash, and maple are darkened to a pleasing color by the 
same treatment. Mahogany is greatly darkened to a 
rich brownish red. The degree of coloring depends 
upon the temperature used and the length of time of 
exposure. The wood must be perfectly dry before 
treatment. An objectionable feature is that it renders 
the wood more absorptive, and it is difficult to finish, 
as it absorbs varnish greatly. The coloration is in no 
wise analogous to a stain, but the effect is produced 
clear through the board. At the same time the hy- 
groscopicity and consequently the swelling and shrink- 
age is reduced by about one-half. The wood, however, 
is made more brittle, although its hardness is not 
greatly reduced. A loss in bending strength of about 
15 per cent, and in work to maximum load (toughness) 
of about 30 per cent, was found in tests made on this 
material. In hardness, however, a general reduction 
of only 9 per cent, was produced by the most severe 
treatment, and some tests on the maple flooring showed 
no loss. On black gum the loss was only 5 per cent. 
A maple floor was laid in which the alternate boards 
were subjected to this treatment and colored almost as 
dark as ebony. It has been in constant use now for 
a year and shows no greater wear of the treated than 
the untreated boards. Its applicability to interior trim 



190 THE KILN DRYING OF LUMBER 

and cabinet work, except where brittleness is detri- 
mental, seems promising, except for the cost of treat- 
ment. The treatment either in dry air or in steam 
does not increase the durability of wood either against 
fungous rot or insect attack, nor is its fire resistance 
increased thereby. 

Darkening wood by fuming with ammonia has been 
practised for many years. It is generally essential 
that the wood contain some acids, as tannic acid in 
oak, in order that the ammonia treatment be effective. 
Red gum sapwood is stained uniformly brown by am- 
monia fuming. This is accomplished in ordinary dry 
kilns. The wood must be thoroughly wet, or must be 
first steamed until wet. Ammonia gas is then forced 
into the kiln at the top, which is closed air tight, and is 
sucked out near the bottom. The fumes are recircu- 
lated by a fan. The wood is left in the fumes for 
several days or longer, according to thickness and the 
effect desired. 



CHAPTEE VIII 

The Impkoved Wateb Speay Humidity Regulated 
Dey Kiln 

It has already been shown that the three funda- 
mental factors necessary in the drying of a pile of lum- 
ber are circulation, humidity, and temperature. In an 
endeavor to produce a commercial kiln in which each 
of these elements could be regulated independently of 
the others, the author designed for the United States 
Forest Service the kiln about to be described. The 
principle of the forced circulation and humidity control 
by means of the sprays of water which was the basis 
of the first patents taken out in 1912 ^ is still the main 
feature of the present kiln, which has been greatly im- 
proved through five years of constant study and ex- 
periment, so that the kiln is now well adapted to com- 
mercial work. Moreover, the recently discovered prin- 
ciple of the downward circulation through the lumber 
pile has been taken advantage of in the newest form 
by so arranging the piles that the air may descend 
diagonally through them into the spray chamber. 

^Patents 1,019,743, March 5, 1912; 1,019,999, March 12, 1912; 963,- 
832, July 12, 1910; 981,818, January 17, 1911. No. 1,228,989, June 
5, 1917. All of these patents are dedicated to the free use of the public. 

191 



192 



THE KILN DRYING OF LUMBER 



GENEKAL DESCRIPTION OF THE KILN 

It is not intended to offer here a working plan or 
specifications, bnt merely to give a sufficient descrip- 
tion to make its eonstrnction and operation plain. For 
best results, each case should always be worked out to 







Fig. 44. — Diagram of the Tiemann Water Spray Humidity Regulated Dry Kiln. (Cross 

sectional elevation.) 

suit the particular requirements, and a design made 
accordingly. For further information the reader is 
referred to the Forest Products Laboratory, U. S. For- 
est Service, Madison, Wis., which is conducted in co- 
operation with the University of Wisconsin. 



IMPROVED WATER SPRAY DRY KILN 193 

In Fig. 44 is shown a cross sectional front elevation 
of this kiln in one of its simplest forms. Spray cham- 
bers, B, B, are placed on the sides. These are six or 
seven feet in height, 12 to 16 inches in width, and extend 
the entire length of the kiln. They are thoroughly 
waterproofed on the sides. The tops of these flues are 
open and may be arranged for a footpath or runway. 
Near the tops are placed the series of sprays, F, F. At 
the bottom are gutters, C, C, which drain to the end of 
the kiln and thence to a well. The bottoms of the flues 
open into the space beneath the heater coils, but the 
air is obliged to pass through zigzag baffle plates, D, D, 
which separate all fine mist from the air but allow the 
air to pass through freely in a saturated condition. 
These baffles may be made up of boards in convenient 
sections. Copper nails or wooden dowels should be 
used. They should fit tightly, as any leakages will 
allow the spray to get through to the steam pipes, which 
would spoil the humidity regulation. H represents the 
heating pipes, which are concentrated towards the 
center. ^ is a floor or shield of loose planks which 
serves as a bottom to walk on and also shields the 
lower course of lumber from direct radiation of the 
heating pipes. The lumber is piled as indicated, with a 
flue in the middle about 12 inches wide. While the in- 
clined pile as illustrated will give the best results, flat 
piling, arranged in the same manner with the flue in the 

13 



194 THE KILN DRYING OF LUMBER 

center, will also work well. With inclined piling the 
boards may be placed solid edge to edge, but with flat 
piling it is advisable to leave cracks between them so 
that the air may descend as it cools, passing through in 
a downward and outward direction. Curtains are hung 
from the roof to the edges of the piles as shown to pre- 
vent the air from passing over the piles and thus 
' ' short circuiting" them. Condensing pipes are placed 
just above the spray chambers at G, G, for use at the 
end of the drying operation when not so great a cir- 
culation is needed. Steam sprays are suitably placed 
beneath the piles for use in removing casehardening. 
The diagram is drawn to scale for a kiln 13 feet wide, 
12y2 feet high, and of whatever length desired. Fig. 45 
shows the arrangement of this kiln for flat piling. 

The water sprays consist of small brass nozzles 
fitted by means of a one-fourth inch pipe ''goose-neck" 
to the supply pipe as shown. The adjustable "ver- 
morel" type of nozzle, such as is used in horticultural 
work,2 which will deliver 2.5 to 3 pounds of water per 
minute at about 45 pounds pressure, has been found 
satisfactory. These should be spaced about three feet 
apart. They should give a spray of water and not a mist. 

The temperature of the water is regulated in a very 
simple manner as follows : 

The water flows by gravity from the gutters into 

*F. E. Myers, Ashland, Ohio. Graduating Vermorel Spray Nozzles. 



IMPROVED WATER SPRAY DRY KILN 195 

a suitable well. From this well it is pumped by a 
suitable small rotary pump direct to a tempera- 
ture regulating, mixing valve.^ Cold water from 



ffa-f/i'''e c/afean 




J- / 'y^feooT jet/pe. 



^>J/orayi 



/'Aesa iaarys are /S ' 

'pa/Kt M- //lay 
^/?ou/</ £e /ifftauei^ 
,ay7e/ froovaa. 



,J^fieafA//i^ 



J>r-ain to u)e/t 



Fig. 45. — Arrangement for flat piling. 



the supply main is also tapped into this valve at ap- 
proximately the same pressure, which should be ad- 
justed to between 30 and 50 pounds. By merely moving 

'Leonard Valve, Leonard-Rooke Co., Providence, R. I. 
Also Powers Regulator Co., Chicago. 



196 THE KILN DRYING OF LUMBER 

a small lever this valve will deliver water at any de- 
sired temperature between the two extremes and hold 
it steady within a degree or two. A steam pipe should 
also be arranged to discharge into the well for heating 
the water when very high humidity is called for. A 
strainer is placed on the water line to prevent the 
sprays from becoming clogged. Should any one be- 
come clogged, however, it can be readily cleaned by its 
small adjusting screw in the top. 

The condensing pipes are also connected to the 
same circuit through a separate valve from the pipe 
line leading from the Leonard temperature regulator, 
so that either sprays or condensers may be operated 
by the same system by merely opening and closing the 
respective valves. Fig. 46 shows diagrammatically the 
arrangement of the spray water and condenser regulat- 
ing apparatus. 

Construction. — ^Any kind of building construction 
may be used, but it should be water- and moisture- 
proof. All cement work except the foundations should 
be waterproofed. This may be accomplished by using 
5 per cent, solution of alum and 8 per cent, of soap in 
the water used for mixing the concrete, or the walls 
may be thoroughly coated with high temperature as- 
phaltum varnish. 

In cold climates either standard wooden studding 
construction or hollow tile is preferable on account of 
insulation. A certain amount of radiation from the 



IMPROVED WATER SPRAY DRY KILN 



197 



side walls is desirable, since it increases the efficiency 
of the condensers and the sprays — the heat given to the 
walls does not need to be removed by the condensers. 
Do not use galvanized pipes or metal in a dry kiln, 
as the fumes rapidly decompose the coating. Use plain 
wrought iron pipes and paint them with a good high 



^nsPfTArs. 




4/un 



3 » Gfi'/tOUir/AG IV/f/W/KX. jr/yf/iir 
f^OZ.Zl£ A^D ATTACM/7£/yr TO 
Ofi£ INCH FEBD PIPE. 

/CC£P iV£l,L rVll. OF iVAT£/i 



Fig. 46, — Diagrammatic sketch of water spray and condenser system. 

temperature asphaltum varnish or black baking japan. 
Operation. — The operation is very simple. The 
heated air rises in the flue between the two piles of 
lumber. As it comes in contact with the piles, parts of 
it are cooled and forced to pass outwardly through the 
piles to the spray chambers. Here the velocity of the 
descending column of air is greatly augmented by the 
sprays. It then passes out through the baffle plates 



198 THE KILN DRYING OF LUMBER 

into the space inunediately beneath the heaters. Here 
it is in a saturated condition and its temperature is 
therefore manifestly the dewpoint of the air after it be- 
comes heated in passing through the steam pipes. This 
may, therefore, be termed the dewpoint method of 
humidity control, since this dewpoint temperature is 
easily controlled by the temperature of the spray water. 

Only two stationary thermometers are necessary 
for determining the humidity and temperature of the 
air entering the lumber, and therefore for operating the 
kiln, one in the baffles at D, which thus records the dew- 
point, and the other in the flue between the piles of 
lumber. No wet bulb is needed, nor any hygrometer. 
It is very convenient to use the recording type of ther- 
mometer ^ having long flexible tubular connection with 
the bulb, and to have both hands recording on the same 
dial. By means of the humidity diagram given in 
Chapter XIV the humidity may thus be quickly de- 
termined at any time. 

The temperature of the entering air may be con- 
trolled by any good form of thermostat,^ of which there 
are many on the market, or by means of a reducing 
valve on the steam line. 

This type of kiln requires very little attention when 
properly operating; once a day, or even once every 
three days, has proved sufficient. 

* Bristol Co., Waterbury, Conn., Gass filled type. 

* Tagliabue Thermometer Co., Bush Terminal, Brooklyn, N. Y., " Per- 
fect" temperature regulator. Powers Regulator Co., Chicago. 



IMPROVED WATER SPRAY DRY KILN 199 

The form of the kiln may be varied from that shown. 
For instance, a single truck may be used. This form 
would be represented by dividing the diagram, Fig. 44, 
vertically into two parts. In another form the spray 
chamber may be placed in the center and the lumber 
flat piled or sloped in the opposite direction. The air 
would then rise next to the side walls and descend in 
the center. For a separate kiln, however, the form illus- 
trated is preferable, on account of the cooling effect of 
the outside walls. Ventilation may also be used, which 
reduces the amount of fresh water required, an out- 
take flue being placed immediately above the spray 
chambers and an intake beneath the heating coils. 

When drying at high humidities it often happens 
that the walls and roof of the kiln are cooler than the 
dewpoint of the air. Wherever this occurs condensa- 
tion will occur and will drip from the ceiling on to the 
lumber, causing stain and interfering greatly with the 
drying. Moreover, the cooling effect upon the air above 
the pile may become so great as to retard the drying 
of the upper portion of the pile. The difficulty may 
be partly overcome by suspending a false ceiling a few 
inches below the ceiling proper. A better method, how- 
ever, and one which entirely eliminates both troubles, 
is to place a few steam pipes along the ceiling, suffi- 
cient only to keep its temperature slightly above that 
of the dewpoint of the air. 



CHAPTER IX 

Dkying by Supeeheated Steam and at Peessuees 
Othee Than Atmospheeic 

1. Superheated Steam. — WTien there is a great deal 
of free water in the wood the quickest possible way to 
get rid of it is merely to heat the material above the 
boiling point, and it will pass off as rapidly as the 
necessary heat is supplied. Not so with the hygro- 
scopic moisture or the moisture in the cell walls. This 
can not be boiled off. Thus wood containing say 60 
per cent, moisture may be reduced to 30, or in some 
cases 25 per cent., as rapidly as heat can be conducted 
to it, but the evaporation will greatly slow up below 
the fiber saturation point. Now at atmospheric pres- 
sure the boiling point is 212°, and it is necessary to 
heat the wood to this point before boiling will take 
place. Exactly the same thing may be accomplished at 
a lower temperature by reducing the pressure to less 
than atmospheric. This is sometimes called the 
'Vacuum process" of evaporation, but it differs in no 
other essential from the boiling process at atmospheric 
pressure than that it takes place at a lower tempera- 
ture and lower pressure. A "vacuum" in itself, as we 
shall see in the chapter on the theory of drying, has no 
inherent or peculiar efi&cacy in producing drying. A 

200 



DRYING BY SUPERHEATED STEAM 201 

dish of water will not evaporate in a vacuum any more 
than in the atmosphere, but it will boil at a lower tem- 
perature. Hence the use of vacuum pans in evaporat- 
ing milk, to reduce the temperature. The most rapid 
method of conveying heat to the lumber is by the use 
of superheated steam, but as soon as it has cooled to 
the boiling point it can produce no evaporation. A 
number of kilns have been constructed for the use of 
superheated steam, both at atmospheric pressure and 
under a partial vacuum. As mentioned elsewhere, the 
evaporative capacity of the superheated steam is pro- 
portional to the product of the quantity coming in con- 
tact with the lumber times the number of degrees it is 
superheated. Consequently, to produce rapid drying 
it is necessary to have a high velocity or a high degree 
of superheat. The chief objections to the use of super- 
heated steam in drying lumber, from the standpoint of 
the process, are the extreme rapidity with which it 
parts with its superheat and becomes saturated and the 
high temperature to which it is necessary to heat the 
lumber. Owing to the former uneven drying is pro- 
duced, for that portion of the pile which first comes in 
contact with the impinging steam dries rapidly, and 
the other portions of the pile, which receive the cooled 
steam, do not dry at all, until they, too, receive the 
superheat after the first portion is dry. By increasing 
the velocity or reducing the size of the pile this dififi- 



202 THE KILN DRYING OF LUMBER 

culty may be lessened. In regard to the temperature, 
it is evident that at atmospheric pressure the wood must 
be heated to 212°. So long as there is plenty of free 
water present it is impossible to heat the lumber above 
this temperature, no matter how hot the steam may be, 
but as soon as the free water has all boiled off, the 
temperature will begin to rise until ultimately it reaches 
the temperature of the superheated steam. Some spe- 
cies, such as the western firs, Alaska cedar, and yellow 
pine, are not seriously injured at this temperature, but 
the majority of woods, including nearly all the hard- 
woods, will not stand it. Some of the conifers as west- 
ern red cedar {Thuja plicata) when heated above the 
boiling point are torn to pieces by the expansion of the 
steam generated in the cells. Temperatures of 300° F., 
or more, are in use with superheated steam, which is 
usually forced into the kiln by means of a blower. The 
heat may be produced, however, by the heating pipes 
within the kiln itself. 

In the improved method recently developed for the 
Forest Service * a remarkably low temperature of 
superheat is made possible by producing a very high 
velocity of the steam passing through the lumber. The 
high velocity is produced within the kiln itself by means 
of steam jets, and the superheating is accomplished by 
steam heating pipes in the current of steam. Very 
successful drying has been accomplished with a tem- 

* See page 48. 




Fig. 47.— Recording thermometer record of temperature of kiln thermostatically 

controlled. 



DRYING BY SUPERHEATED STEAM 



203 



perature as low as 220° to 225° F. (See Table IV, 
page 50.) 

Application of New Method. — In applying this 
method, it can be readily combined with my spray-con- 
denser system, by the addition of the water sprays and 
the pipe condensers in the spray chambers. By this 
combination, the drying may be hastened towards the 
end of the operation without necessitating an exces- 




FiG. 48.— Simple arrangement for use of High Velocity Superheated Steam Method 
with inclined piling system. S,S, steam jets, ff, heating coils. C,C, curtains. 

sively high temperature to end with. This is desirable 
also where very dry wood is required. 

In the illustrations the water sprays and condensers 
are not shown. In Fig. 48 the condensers would be 
placed on the side walls, immediately above the steam 
jets S,8, and the water sprays alongside the steam jet 
pipe. In Fig. 49 the condensers could be placed be- 
neath the lumber, where the heating pipes are shown. 
There is no place, however, for the water sprays in this 
arrangement. 



204 



THE KILN DRYING OF LUMBER 



Flat Piling with Reversible. Circulation. — The most 
recent design for tlie use of this high velocity low super- 
heat method is shown in Fig. 50, in which the lumber is 
flat piled.^ The heating pipes are on the sides of the 
kiln, and an auxiliary system of pipes is placed between 
the two stacks of lumber to restore the superheat of the 
steam in its horizontal passage through the lumber. 
The steam jets are placed in the free passageways both 




Fig. 49. — Arrangement for use of High Velocity Superheated Steam Method with edge 
piling system. S,S, steam jets. H,U, heating coils. 

beneath and above the piles. By the use of two sets of 
jets facing in opposite directions a great advantage is 
obtained in being able to reverse the direction of the 
circulation during the drying operation, thus hastening 
it and producing more uniform results. 

Other methods have been tried from time to time, 
such as placing the lumber in a cylinder lined with heat- 

^ Patent applied for and dedicated to tlie public for free use. 



DRYING BY SUPERHEATED STEAM 



205 



ing pipes and drawing a vacuum. A small number of 
boards can be rapidly dried by this method, since they 
receive heat from the cylinder walls by direct radiation. 
With a large pile of lumber, however, this is not the 
case, aud it is impracticable to get heat to the inside of 
the pile, which is consequently very slow in drying. The 



**?." 




Fig. 50. — Arrangement for use of the New High Velocity Low Superheat Method 
with flat piled lumber and with reversible circulation. 

only advantage the vacuum has is in reducing the boil- 
ing temperature of the water in the wood. More heat, 
however, is required to vaporize water at a low tem- 
perature than at a high temperature, as its latent heat 
is greater. Still another method which has been tried 
consists in heating the wood in saturated steam, then 



206 THE KILN DRYING OF LUMBER 

drawing a vacuum, again heating in steam, and drawing 
a vacuum, and thus repeating the operation until the 
wood has dried to the desired point. The principle 
upon which this depends is that the specific heat of the 
wood substance plus that of the water itself contained 
in the wood after heating in the steam supplies suffi- 
cient heat to vaporize a certain amount of water when 
the vacuum is drawn. When the wood has cooled to 
the saturation point corresponding to the vacuum pres- 
sure, it must again be heated to produce any further 
drying. The amount which the wood will dry can be 
readily calculated, if the percentage of moisture it con- 
tains is known. Thus, suppose we have 1 pound of dry 
wood and 50 per cent, or half a pound of water. It is 
heated to 212°, and a vacuum of 15 inches is drawn 
(absolute pressure of 15 inches), which corresponds to 
a boiling point of about 179° Fahrenheit. There is then 
212-179^33° of heat available for evaporation. As 
the specific heat of wood is 0.33, the total available heat 
in the pound and a half of wet wood is (0.33+0.50) X33° 
= 27.39 heat units, and this is capable of evaporating 
27.39 =,0.0277 pound of water, or the moisture in the 

969 

wood will be reduced from 50 per cent, to 47.2 per cent, 
by one heating and vacuum. A second time it will be 
reduced accordingly and so on. After a while, how- 
ever, the absorption during the steaming will equal the 
loss during the vacuum. It is then possible to continue 



DRYING BY SUPERHEATED STEAM 207 

the drying by heating in partially humidified air, with 
sufficient humidity to prevent evaporation during the 
heating process, followed as before by a vacuum. Theo- 
retically, this system of drying should be as near per- 
fection as it is possible to obtain, because no drying 
takes place during the heating process, but it is all pro- 
duced from the self-contained heat only and proceeds 
from the inside outwardly, the free water being driven 
from the hot interior to the cooler surface. Instead of 
drawing a vacuum the same or nearly the same result 
is accomplished by heating and then exposing to cool 
dry air. In this case, the operation is also self -regula- 
tory, since those parts receiving the greatest air circula- 
tion are cooled more rapidly and evaporation is re- 
tarded. Practically, it does not work very satisfac- 
torily, as it requires too long a time to dry and involves 
too many processes ; it is cumbersome to handle. Some 
woods, as oak, surface check badly by this method along 
the medullary rays. 

2. Steaming Under Pressure. — Steaming lumber for 
various lengths of time under pressures greater than 
atmospheric has frequently been tried as a preliminary 
treatment to both kiln drying and air drying. This is 
usually accomplished in a steel cylinder into which the 
entire truck load of lumber may be run. 

Statements as to results obtained by this treatment 
are very conflicting. Some claim that green lumber 



208 THE KILN DRYING OF LUMBER 

can be air dried to shipping weight after steaming at 20 
pounds for twenty minutes in 30 days, which would 
otherwise require three months. Others claim that 2- 
inch black walnut and mahogany can be kiln dried to 
shipping weight in a dry kiln in two days after steam- 
ing at 5 pounds for 2 hours. Again some have had 
very poor success and have found that it tears the lum- 
ber to pieces. Air-dried oak flooring is being handled 
by this method at 20 pounds for 10 minutes in at least 
one case with apparent success. In the majority of 
cases, however, steaming under pressure has been 
found to be very harmful to oak, due to surface check- 
ing, which takes place as soon as the oak is removed 
from the steam. 

Laboratory experiments and actual moisture tests 
do not bear out all of the claims for this treatment. The 
great increase in rate of drying has not been found to 
exist, although a certain advantage in point of time of 
drying, no doubt, may be obtained in some cases with 
green wood. The following deductions appear possible 
concerning the effect of preliminary steaming under 
pressure : 

(1) The wood is heated in the quickest possible 
time. Air dry wood will take on moisture, but green 
wood will not, if the lumber pile is arranged so that 
the condensation may drip off into the bottom of the 
cylinder. 



DRYING BY SUPERHEATED STEAM 209 

(2) When removed from the cylinder the self-con- 
tained heat of tlie wood is capable of causing spon- 
taneously the vaporization of a definite amount of 
moisture. As explained above, in the case of steaming 
at 212°, take for example green wood containing 50 
per cent, moisture, then for each pound of dry wood 
there is half a pound of water present. Suppose this 
wood be heated in saturated steam at 20 pounds pres- 
sure until it is thoroughly heated through, then taken 
out in the air and allowed to cool. Its temperature 
when removed from the cylinder will be 259°, and if it 
cools to 70° there will be (1 X 0.33 + 0.50) X (259 - 70) 
= 156.9 heat units available for evaporating moisture. 
This is capable of evaporating 156.9-^966 = 0.162 
pound of water, or the wood may be reduced from 50 
per cent, to nearly 34 per cent, moisture as soon as it 
cools. This will give quite a start over the unsteamed 
material. Beyond this there appears to be no gain in 
rate of drying, the drying curves for steamed and un- 
steamed wood being nearly identical. 

Experiments made on carefully matched blocks of 
red oak 2x2x6 inches in size gave the following sur- 
prising results : 

Steamed 1 hour at 20 pounds the steamed pieces 
dried faster than the unsteamed. 

Steamed 1 hour at 40 pounds the two drying curves 
were practically identical. 

]4 



210 THE KILN DRYING OF LUMBER 

Steamed 1 hour at 60 pounds the steamed pieces 
dried considerably slower than the unsteamed. 

Steamed 1 hour at 80 pounds, the steamed pieces 
dried decidedly slower than the unsteamed. 

A 4-inch disk was sawed across a freshly felled tree 
of basswood, containing 89 per cent, moisture. This 
was split in two across the middle, leaving the bark in- 
tact on both pieces. Half of it was steamed at 20 
pounds for 20 minutes. The two halves were then 
stood edgewise on a table in a heated laboratory 
(winter) and weighed daily. The steamed piece took 
on a little moisture during the steaming, but rapidly lost 
this and came back to its original condition. The two 
drying curves were identical for a time, then the 
steamed piece dried a little less rapidly than the un- 
steamed. A remarkable fact, however, was observed, 
namely, that the unsteamed piece developed quantities 
of small radial surface checks or crackles as soon as it 
passed 30 per cent, moisture, or about its fiber satura- 
tion point, whereas the steamed piece showed scarcely 
a check even when dried down to considerably below the 
point where the unsteamed piece began to crackle very 
badly. The explanation of this appears to be that in 
the steamed piece the drying took place from within 
outwardly, due to the heat in the interior driving the 
water from the center to the surface, whereas the un- 
steamed piece dried most rapidly from the surface, 



DRYING BY SUPERHEATED STEAM 211 

causing the surface to dry and shrink while the center 
still contained some free water. Other experiments 
made on boards of western larch, oak, red gum, indicate 
no gain in the rate of drying, but a considerable saving 
in time due to the initial loss produced by the self- 
contained heat. 

(3) There appears to be a decidedly beneficial re- 
sult in some cases in the distribution of moisture, as 
explained above, in the case of the basswood disk, dur- 
ing the drying. The internal heat appears to force the 
moisture from the interior towards the surface, and the 
boards dry from within outwardly. 

(4) Internal stresses are relieved by the high tem- 
perature and the wood adjusts itself. Thus crooked 
boards may be straightened by this treatment if held 
flat while in the steam. 

(5) It is possible that in some cases shrinkage may 
be reduced due to the relieving of internal stresses in 
drying, although direct experimental evidence is con- 
flicting in this respect. 

(6) The color is darkened, particularly that of the 
sapwood. 

(7) Eesins and gums are hardened and rendered 
less liable to give trouble in finishing the wood, and 
fungous growth or rot is temporarily killed. There is 
no evidence, however, that the durability of the wood 
is increased. 



212 THE KILN DRYING OF LUMBER 

(8) Different species behave very differently. 
Where good results may be obtained as in red gum, 
basswood, ash, and many of the softer woods, injury 
may be done to many of the hard dense woods, as oak 
and probably black walnut. The oak when removed 
from the steam almost invariably opens up in innumer- 
able small surface checks along the medullary rays. 
This is particularly visible on bastard sawed (flat 
grain) boards. See Fig. 51, which shows a piece of red 
oak soon after removal from the steam. 

(9) The strength of the wood is reduced, depend- 
ing upon the pressure, length of time exposed in the 
steam, and the species of wood. 

It is commonly said that steaming "opens the 
pores" of the wood. This is a misleading statement, 
as microscopic examination shows that no material 
change is produced in the structure of the wood by the 
steaming. In the case of woods which contain ' ' tyloses ' * 
in the pores or vessels, these cells are still intact after 
steaming. For example, white oak, which is completely 
sealed by these ingrowths known as "tyloses" so that 
air and liquids can not be forced through the green 
wood, still remains in the same condition after steam- 
ing. It is not changed thereby into a wood like red 
oak, in which the pores are all open in the green log. 
In the case of gums and resins, the steaming melts and 
distributes them throughout the wood. Thus the sap- 



s 

I? 

Si 



o5 



B 3 






T to 




DKYmG BY SUPERHEATED STEAM 213 

wood of yellow pine may be rendered more porous by 
the melting of the resin from out the resin ducts, and in 
the case of black walnut the coloring matter is dis- 
tributed. Chemical changes are brought about in the 
sap in the sapwood. It is also probable that a partial 
hydrolysis of the cell walls takes place even at a tem- 
perature of 212°. At about 20 pounds gauge pressure, 
259° F., a distillation of the wood begins to take place, 
and acid fumes are given off. At higher temperatures 
more and more chemical action occurs until finally com- 
plete distillation of the wood is brought about. 

Steaming will relieve casehardening, as has been ex- 
plained in Chapter IV, particularly in the case of air- 
dried wood, by softening the surface and allowing the 
adjustment of internal stresses to take place. Steam- 
ing at temperatures less than 212° is frequently re- 
sorted to for this purpose during the progress of kiln 
drying. When the surface dries so rapidly that the 
continuity of flow of moisture from the interior to the 
surface has become interrupted, thus causing a cessa- 
tion in the rate of drying, it is commonly believed that 
steaming will start the drying again, by moistening the 
surface and reestablishing the continuity. While this 
theory appears plausible, there are no data available to 
establish it as a fact. 

A carefully conducted research as to the effect of 
steaming upon the rate of drying, shrinkage, and physi- 



214 THE KILN DRYING OF LUMBER 

cal condition of 2-incli post oak, Biltmore ash, western 
larch, and Noble fir, both green and air dried, was 
carried out in 1915 at the Forest Products Laboratory 
by Mr. J. E. Imrie. Blocks of the various species 2" X 
4"' and 2" X 6", 6" long, and boards 2" X 4" and 2'' X 
6" and 30" long were used, making 120 carefully 
matched pieces in all. In every case, each treated piece 
had a counterpart which was air dried without treat- 
ment, together with the corresponding piece after treat- 
ment. The treatments were as follows : 

A steamed 10 minutes at 20 pounds gauge. 
B steamed 15 minutes at pound gauge. 
C steamed 4 hours at 20 pounds gauge. 
D steamed 24 hours at pound gauge. 

All pieces were weighed periodically while air dry- 
ing and their drying rate curves plotted. There is no 
very marked difference in the rate of the drying be- 
tween the treated and the untreated specimens, but in 
some instances there is an immediate loss, which thus 
reduces the initial moisture content, and if the times 
required to reach a given moisture condition of 20 per 
cent, be compared there is a slight gain with most of 
the steamed pieces. Grreen oak from treatments C and 
I) reached 20 per cent, moisture in about two-thirds the 
time required by the unsteamed pieces. 

The effect upon the shrinkage is even more erratic: 
in some cases it was greater, in others less. In the 



DRYING BY SUPERHEATED STEAM 215 

case of the Biltmore ash shrinkage was reduced in 
almost every case, but in the green block treatment C 
it was enormously increased. In post oak the shrink- 
age was generally increased, except for the air dry- 
boards, in which it was decreased. In western larch it 
appears to have been somewhat decreased, except in 
green blocks, and in Noble fir it was increased in the 
boards. 

As to the physical condition, treatments A and B 
did not harm the green oak boards, and B did not harm 
the air-dried oak. C and D were very detrimental to 
oak in checking and warping. A, C, and D injured 
the air dry oak. In the ash only D caused any de- 
grading. In the larch and the fir the green boards 
were not injured by any of the treatments, but all of 
the air dry boards were degraded. 



CHAPTER X 

THEOEETICAIi CoiTSIDERATIONS AND CALCULATIONS, Hu- 

MiDiTY, Evaporation, Density, the Drying Cycle, 
Amount of Air and Heat Required, Thermal 
Efficiency 

elementary principles of drying 
Before taking up the theoretical discussion, a few 
remarks upon the elementary principles of drying will 
be of assistance. 

EVAPORATION REQUIRES HEAT 

In the first place, it should be borne in mind that it 
is the heat which produces evaporation and not the 
air nor any mysterious property assigned to a 
''vacuum." For every pound of water evaporated at 
ordinary temperatures, approximately one thousand 
British thermal units of heat are used up or *' become 
latent," as it is called. This is true whether the evapo- 
ration takes place in a vacuum or under a heavy air 
pressure. If this heat is not supplied from an outside 
source, it must be supplied by the water itself (or the 
body being dried) and its temperature will consequently 
fall until the surrounding space becomes saturated with 
vapor at a pressure corresponding to the temperature 
which the water has reached; evaporation will then 
216 



CONSIDERATIONS AND CALCULATIONS 217 

cease. The pressure of the vapor in a space saturated 
with water vapor increases rapidly with increase of 
temperature. At a so-called vacuum of 28 inches, 
which is about the limit in commercial operations, and 
in reality signifies an actual pressure of two inches of 
mercury column, the space will be saturated with vapor 
at about 101 degrees Fahrenheit. Consequently, no 
evaporation will take place in such a vacuum unless the 
water be warmer than 101 degrees, provided there is no 
air leakage. The qualification in regard to air is neces- 
sary to be exact, for the following reason : In any given 
space the total actual pressure is made up of the com- 
bined pressures of all the gases present. Now if the 
total pressure (''vacuum") is two inches, and there is 
no air present, it is all produced by the water vapor 
(which saturates the space at 100 degrees) ; but if some 
air is present and the total pressure is still maintained 
at two inches, then there must be less vapor present, 
since the air is producing part of the pressure, so that 
the space is no longer saturated at the given tempera- 
ture. Consequently, further evaporation may occur, 
with a corresponding lowering of the temperature of 
the water until a balance is again reached. Without 
further explanation it is easy to see that but little 
water can be evaporated by a vacuum alone, and that 
the prevalent idea that a vacuum can of itself produce 
evaporation is a fallacy. If heat be supplied to the 



218 THE KILN DRYING OF LUMBER 

water, either by conduction or radiation, evaporation 
will take place in direct proportion to amount of heat 
supplied, so long as pressure is kept down by the pump. 

At 30 inches of mercury pressure (one atmosphere) 
the space becomes saturated with vapor and equilib- 
rium is established at 212 degrees Fahrenheit. If heat 
be now supplied to the water, however, evaporation 
will take place in proportion to the amount of heat 
supplied, so long as the pressure remains that of one 
atmosphere, just as in the case of the vacuum. Evapo- 
ration in this condition where the vapor pressure at 
the temperature of the water is equal to the gas pres- 
sure on the water is what is commonly called by the 
term of boiling, and the saturated vapor entirely dis- 
places the air. "Whenever the space is not saturated 
with vapor, whether air is present or not, evaporation 
will take place, by boiling if no air be present or by 
diffusion under the presence of air, until an equilibrium 
between temperature and vapor pressure is resumed. 

Relative humidity is simply the ratio of the actual 
vapor pressure present in a given space to the vapor 
pressure when the space is saturated with vapor at the 
given temperature. It matters not whether air be pres- 
ent or not. One hundred per cent, humidity means that 
the space contains all the vapor which it can hold at the 
given temperature — it is saturated. Thus at 100 per 
cent, humidity and 212 degrees Fahrenheit the space is 



CONSIDERATIONS AND CALCULATIONS 219 

saturated, and since the pressure of saturated vapor at 
this temperature is one atmosphere, no air can be pres- 
ent under these conditions if open to the atmosphere. 
If, however, the total pressure at this temperature 
were 20 pounds (5 pounds gauge), then it would mean 
that there was 5 pounds air pressure present in addi- 
tion to the vapor, yet the space would still be saturated 
at the given temperature. If, however, the tempera- 
ture were 101 degrees Fahrenheit, the pressure of satu- 
rated vapor would then be only one pound, and the 
additional pressure of 14 pounds, if the total pressure 
be atmospheric, would be made up of air. In order to 
have no air present and the space still saturated at 
101 degrees, the total pressure must be reduced to one 
pound by a vacuum pump. Fifty per cent, relative 
humidity, therefore, signifies that only half the amount 
of vapor required to saturate the space at the given 
temperature is present. Thus at 212 degrees tempera- 
ture the vapor pressure would be only 7^^ pounds 
(vacuum of 15 inches gauge). If the total pressure 
were atmospheric, then the additional 7>< pounds is 
simply air. ''Live steam" is simply saturated water 
vapor at a pressure usually above atmospheric. We 
may just as truly have live steam at pressures less than 
atmospheric, at a vacuum of 28 inches for instance. 
Only in the latter case its temperature would be lower, 
viz., 101 degrees Fahrenheit. Superheated steam is 



220 THE KILN DRYING OF LUMBER 

nothing more than water vapor at a relative humidity- 
less than saturation, but is usually considered at pres- 
sures above atmospheric, and in the absence of air. 
The atmosphere at say 50 per cent, relative humidity 
really contains superheated steam or vapor, the only 
difference being that it is at a lower pressure and tem- 
perature than we are accustomed to thinking of in 
speaking of superheated steam, and it has air mixed 
with it to make up the deficiency in pressure below the 
atmosphere. 

Two things should now be clear: that evaporation 
is produced by heat and that the presence or absence of 
air does not influence the amount of evaporation. It 
does influence the rate of evaporation, however, as it is 
retarded by the presence of air. The main things in- 
fluencing evaporation are: first, the quantity of heat 
supplied, and, second, the relative humidity of the im- 
mediately surrounding space. 

IMPORTAIJCE OF CIECULATIOIT 

A piece of wood may be heated in three ways: (1) 
By convection of the air and vapor or other gases ; (2) 
by conduction through some body in contact therewith ; 
(3) and by radiation. It is evident upon a little con- 
sideration that of these three ways only the first one is 
ordinarily available for use in heating a pile of lumber, 
since only the outside surface of the pile could be heated 



CONSIDERATIONS AND CALCULATIONS 221 

by the other two methods; hence, the necessity of a 
large and thorough circulation of air. Drying in a 
vacuum would be feasible, only providing there were 
some means of conveying the heat to the wood. A 
single stick can be readily dried in a vacuum, as it can 
receive heat on all sides by radiation from the walls of 
a steam- jacketed cylinder, but this is impracticable 
when it comes to any quantity of lumber except in the 
case of superheated vapor alone, as will be shown later, 
since only the outer surface of the outside boards would 
receive the heat in this way, and the inside ones would 
not dry. Thus, by drawing a vacuum the means of heat- 
ing the wood are reduced. Later on it will be shown, 
however, that drying at low pressure in absence of air 
should give the highest theoretical heat efficiency, but 
the volume of vapor required is excessive. 

KATE OF EVAPOBATION CONTROLLED BY HUMIDITY 

It is essential, therefore, to have an ample supply 
of heat through the convecting currents of the air., 
But in the case of wood the rate of evaporation must 
be controlled, otherwise checking will occur, due to too 
rapid surface drying, the surface drying more rapidly 
than the moisture is transmitted through the wood 
itself. This factor can evidently be completely con- 
trolled by means of the relative humidity. It is clear 
now that when the air, or, more properly speaking, the 



222 THE KILN DRYING OF LUMBER 

space, is completely saturated no evaporation can take 
place at the given temperature. By reducing the humid- 
ity evaporation takes place more and more rapidly. 
Another bad feature of an insufficient and non-uniform 
supply of heat is that each piece of wood will he heated 
to the evaporating point on the outer surface, the in- 
side remaining cool until considerable drying has taken 
place from the surface. Ordinarily in dry kilns high 
humidity and large circulation of air are antitheses to 
one another. To obtain the high humidity the circula- 
tion is either stopped altogether or greatly reduced, 
and to reduce the humidity a greater circulation is in- 
duced by opening the ventilators or otherwise increas- 
ing the draught. This is evidently not good practice, 
but as a rule unavoidable. The humidity should be 
raised to check evaporation without reducing the cir- 
culation. In the new kiln the humidity is regulated 
without greatly checking the circulation. 

ELEMENTAEY PRINCIPLES OP HYGROMETEY 

Relative Rwrnidity and Dewpoint. — It is necessary 
to understand hygrometry in order to get an intelligent 
idea of drying operations. As stated before, at any 
given temperature it requires the same quantity of 
water vapor to saturate a given space whether there be 
any air present or not, and the pressure of the vapor 
is the same in both cases. The total pressure (as regis- 



CONSIDERATIONS AND CALCULATIONS 223 

tered by tlie gauge) will not be the same, however, since 
if air be present its pressure is added to that of the 
vapor. It is really the space and not the air which is 
saturated. For instance, at 101 degrees Fahrenheit it 
takes about 20 grains of vapor to saturate a cubic foot 
of space. If no air be present there will be a pressure 
of vapor only, which will be about one pound or a 
vacuum of 28 inches. If this be open to the atmos- 
phere then the air will rush into the space until the 
total pressure will be one atmosphere or about 15 
pounds. There will then be one pound of pressure 
produced by the vapor, as before, and 14 pounds of air 
pressure. The space will still be saturated, if the tem- 
perature is kept at 101 degrees. If this be heated now 
to 160 degrees and open to the atmosphere so that the 
pressure is kept constant, the ratio of pressures of 
vapor and air will remain the same ; there will still be 
one pound due to vapor and 14 pounds due to the air. 
(The weights in the cubic foot of space of both, how- 
ever, will decrease, due to expansion by heat.) At 160 
degrees, however, it requires 91 grains of vapor to satu- 
rate a cubic foot of space and its pressure is nearly five 
pounds (absolute). Consequently, the relative humid- 
ity at 160 degrees of this space will be ys or 20 per cent. 
Conversely, if this air and vapor at 20 per cent, rela- 
tive humidity and 160 degrees temperature be cooled to 
101 degrees, all at the same atmospheric pressure, the 



224 THE KILN DRYING OF LUMBER 

space will again become saturated and any further cool- 
ing will cause precipitation or condensation. This is 
called the dewpoint, that is, 101 degrees is the dew- 
point of air with 20 per cent, humidity at 160 degrees. 
In Chapter XIV a humidity diagram is given for solv- 
ing all problems of this nature. The concave curves on 
this diagram are simply curves of constant vapor pres- 
sure with change of temperature and relative humidity, 
and the grains of vapor per cubic foot, at saturation or 
the dewpoint, are given in numerical figures. From 
this it is seen that the dewpoint determines the relative 
humidity when the temperature is raised, or vice versa. 
If we take saturated air at known temperature and heat 
it up any given desired amount, the resulting relative 
humidity is thereby determined. This is the principle 
upon which the humidity regulation depends in the new 
kiln designed by the vnriter.^ It is also evident that 
whenever air is cooled below its dewpoint condensation 
takes place. This is the principle of the condenser. 
There are a number of kilns which have made use 
of this principle to dry the air. Pipes are used for the 
condensers, and cold water is circulated through the 
pipes. The same thing can be accomplished by a spray 
of cold water in place of the pipes, provided all the fine 
mist be subsequently removed from the air, or even by 
a surface of cold water. In the new kiln a fine spray of 

* For a description of this kiln see Chapter VIII. 



CONSIDERATIONS AND CALCULATIONS 225 

water is used instead of a condenser, which has the ad- 
ditional advantage that when the water is heated above 
a certain temperature (temperature of the wet bulb 
in a wet-and-dry-bulb thermometer) it will humidify 
the air. By simply changing the temperature of the 
spray the air may be supplied at any desired humidity. 

THEORETICAL DISCUSSION OF EVAPORATION 

In considering the drying effect of vapor alone 
(superheated steam) and of air mixed with the vapor, 
one very significant fact must be noticed. Saturate 
vapor alone in cooling and in order to remain saturate 
must absorb heat. Its specific heat is negative, so that 
the only way it can impart heat to a body is by con- 
densation. It is, therefore, incapable of producing 
evaporation. When air is present with the saturate 
vapor, however, the air can supply some of this heat, 
according to the pressure of the air present, so there 
will be less condensation. Still more important is the 
fact that when air is present with the vapor sufficient 
heat can be supplied to the body being dried by means 
of the air without greatly superheating the vapor, thus 
keeping a high relative humidity and at the same time 
supplying a sufficient amount of heat to carry on the 
evaporation. With vapor alone (superheated steam) 
a relatively high degree of superheating, which means 
a correspondingly low relative humidity, is required in 
practice in order to supply the necessary heat for evapo- 
15 



226 THE KILN DRYING OF LUMBER 

ration, after the material has become heated through to 
the temperature of the saturated vapor at the pressure 
used. Remember that the temperature of the wet wood 
corresponds to that of the wet bulb in the hygrometer 
when air is present, but very nearly to the dewpoint in 
the presence of superheated vapor alone. 

Evaporation in the Absence of Air. — In vapor alone, 
no air being present, evaporation from a surface of 
water takes place at the dewpoint, but when the water is 
intimately contained in other substances the tempera- 
ture must be higher than the dewpoint. If air is pres- 
ent it retards the rate of evaporation from a free sur- 
face of water, so that the surface is warmer than the 
dewpoint, depending upon the degree of relative humid- 
ity in the air. In the case of a substance like wood, 
while its surface is wet its temperature will not rise 
above that of the wet bulb in the presence of air, nor 
above the dewpoint in superheated vapor alone. As it 
becomes drier, however, its temperature wiU rise, due 
to its affinity for retaining moisture. In the former 
condition there is no danger of too rapid drying, but in 
the latter condition great danger arises, and if the 
humidity is too low or the superheat is too high the 
temperature will rise and the drying may become too 
rapid from the surface so that the moisture is not 
transmitted from the center as rapidly as it is removed 
from the surface and casehardening results. 



CONSIDERATIONS AND CALCULATIONS 227 

In considering the manner in which drying takes 
place in superheated steam, it may be looked upon in 
this way. Suppose the pressure is atmospheric and 
that a wet piece of wood has been heated in saturated 
steam to 212 degrees. No evaporation will take place 
until additional heat be added. Now suppose steam 
superheated to 232 degrees or 20 degrees of superheat 
be introduced. The portion immediately in contact 
with the surface of the wet wood will be cooled to 212 
degrees, and in so doing it will vaporize a certain por- 
tion of water from the surface. As the specific heat of 
this steam is, speaking in round terms, one-half, and as 
it requires about 1000 thermal units to vaporize one 
unit of water, let us consider a single molecule of water 
at 212 degrees. To vaporize this molecule of water 
will therefore require contact of one hundred of the 
molecules of superheated steam at 232 degrees. We 
will then have one hundred and one molecules of steam 
in the saturated condition at 212 degrees, and have 
evaporated one molecule of water. Evaporation must 
then cease at this point unless this saturated steam be 
replaced by some fresh superheated steam. Evapora- 
tion from a free surface of water in the absence of air 
(in superheated steam) always takes place at the boil- 
ing point (which in this case is the same as the dew- 
point). If, however, there is a deficiency of water in the 
wood, then it requires more heat to separate this water 



228 THE KILN DRYING OF LUMBER 

from the wood and to vaporize it, and the evaporation 
will take place at a higher temperature than the dew- 
point. In fact, evaporation may cease altogether in the 
superheated steam, and a higher degree of superheat- 
ing be required (which is equivalent to a lower humid- 
ity) to get the moisture out of the wood. In the former 
case of a surface of free water the rate of evaporation 
depends entirely upon the amount of heat transmitted 
to the water, whether by increasing the circulation or by 
increasing the degrees of superheat, it matters not, the 
result is the same. In the latter case, when the moist- 
ure is intimately contained in the wood, however, the 
rate depends largely upon the relative humidity. (In 
using the term relative humidity as applied to super- 
heated steam, it is understood to mean the ratio of the 
actual vapor pressure to that of the pressure of satu- 
rated vapor at the given temperature, as explained 
before.) There is a balance between what might be 
termed the retentive or attractive property of the wood, 
"hygroscopicity," and the tendency of the moisture to 
vaporize. It is the difference between the tension of 
the vapor at the higher temperature of the wood and 
the tension actually existing in the space surrounding 
the wood. This retentive property increases as the 
wood becomes drier and decreases as it approaches 
the wet condition. Experiments indicate that it is, 
generally speaking, nearly inversely proportional to 



CONSIDERATIONS AND CALCULATIONS 229 

the amount of moisture remaining in the wood. 

Evaporation When Air is Present. — ^When air is 
present with the superheated steam or water vapor, the 
conditions are quite different. Vaporization of a par- 
ticle from the surface of the free water is retarded by 
the air pressure, so that the temperature of the water 
may be raised above the dewpoint. (In reality what 
probably happens is that the layer of air in immediate 
contact with the water becomes saturated and has a 
higher vapor pressure corresponding to the tempera- 
ture of the surface of the water, and the air retards the 
diffusion of this vapor. The temperature of the water, 
however, can not, of course, exceed the boiling point 
for the given pressure, at which point the conditions 
must become the same as those for superheated steam 
alone just discussed, since then the air is entirely dis- 
placed by the water vapor.) 

The air now as well as the vapor conducts heat to 
the water so that the rate of evaporation at given pres- 
sures depends in this case not alone on the quantity of 
heat supplied (by circulation and degrees of superheat- 
ing), but upon the relative amounts of vapor and air 
present. That is to say, the lower the relative humid- 
ity the greater is the rate of evaporation at a given tem- 
perature and pressure.2 The temperature of the water 



* The following equation is given by Dr. Julius Hann in his " Lehr- 
buch der Meteorologie " for the rate of evaporation from a surface of 
free water: 



230 THE KILN DRYING OF LUMBER 

will correspond to that of the wet bulb and not that of 
the dewpoint. When the wood becomes partially dried 
its temperature will rise as in the case of superheated 
steam, and it may be heated even above the boiling 
point at the given pressure without giving up all of its 
moisture, provided there be some vapor in the air. 

Conclusion as to Drying in Vapor Alone and in Air 
and Vapor. — Thus it is seen that in the case of moist 
air the relative humidity is of prime importance and 
the rate of drying may be controlled by the relative 
humidity provided there be sufficient circulation to 
supply the heat required. In the case of steam alone, 
the rate of drying as just shown depends upon the 
quantity of circulation as well as the degree of super- 
heating. Hence the conclusion follows that moist air 
with ample circulation should give more uniform dry- 

v=o {l + at) (E — e) 

;tp = velocity of evaporation. 

c = constant for any given mean atmospheric pressure B. 

T> 

c becomes c —for another barometric pressure 6. 
b 
a = coefficient of air expansion, 

* = temperature of the air. 
E = maximum vapor pressure at the temperature t. 

e=aetual vapor pressure in the air. 
i(7 = velocity of the air over water. 

This signifies that, other factors being the same, the rate of evapo- 
ration increases inversely with the barometric pressure; inversely as 
the density at the given temperature; directly as the difference in vapor 
pressure between that at saturation and the actual pressure present in 
the air; and directly as the square root of the velocity of the air over 
the surface. 



CONSIDERATIONS AND CALCULATIONS 231 

ing throughout than superheated steam, which varies 
with the rate of circulation in each portion. 

But the chief difficulty with superheated steam at or 
above atmospheric pressure is the high temperatures to 
which the material must be subjected, the minimum 
with very wet wood being 212 degrees, and increasing 
as the wood dries. Below atmospheric temperatures, it 
is not only costly to construct apparatus for operating 
at a vacuum, but the heating medium is attenuated, 
requiring an excessive volume of vapor to be circulated, 
or else the danger incurred by excessive degree of 
superheating, that the wood as it becomes dry on the 
surface will be heated too high, as in the case of steam 
at atmospheric pressure. Instead of using a vacuum 
with superheated vapor, the same result so far as the 
vapor is concerned can be obtained by letting air be 
combined with the vapor, in which case the air makes 
up the deficiency of pressure and atmospheric pres- 
sure can be used instead of the vacuum. For instance, 
consider a vacuum of 28 inches, which is about the ex- 
treme in mechanical operations ; this will give an abso- 
lute vapor pressure of about one pound and a tempera- 
ture of 102 degrees Fahrenheit for saturated condi- 
tions. Precisely the same value for the vapor occurs if 
saturated air at 102 degrees Fahrenheit and atmos- 
pheric pressure be used instead, in which case the ad- 
ditional heating capacity of the air present is available 



232 THE KIEN DRYING OF LUMBER 

also. There would then be in a cubic foot of space 
one pound of vapor pressure and 13.7 pounds of dry 
air pressure. This amount of vapor would weigh 1/334 
or .0030 pound, and the air 1/15.2 or .0658 pound (15.2 
being the volume of one pound of dry air at 13.7 pounds 
pressure and 102 degrees temperature). 

Heating Capacities of Air and Vapor in Mixture. — 
The heating capacity of the vapor in this cubic foot of 
space in falling one degree from 103 degrees to 102 de- 
grees is .003 X .42 3 = .00126 B.t.u. as before, while 
that of the air present is .0658 X .237 =^ 0.156 B.t.u., 
or more than ten times that of the vapor present. The 
total heating capacity of one cubic foot of the mixture 
in falling one degree from 103 degrees to 102 degrees 
is then the sum of these two, viz., .01686 B.t.u. The 
latent heat of evaporation at 102 degrees being 1043, 
it will require the heat given up by 1043/.01686 = 61,862 
cubic feet of the mixed air and vapor falling one degree 
from 103 degrees to saturation at 102 degrees, which is 
very much less than that required for vapor alone, 
which, as will be shown further on, is 829,433 cubic feet. 
In fact, the quantity in volume is less than that of dry 
air alone at 212 degrees and one atmospheric pressure 
(69,000) as figured further on. If the vapor be super- 
heated say to 112 degrees, its pressure remaining the 

® The specific heat of superheated vapor at this temperature is 0.421 
as given by Thiesen. 



CONSIDERATIONS AND CALCULATIONS 233 

same as before, tliis is simply equivalent so far as the 
vapor is concerned to air at atmospheric pressure with 
a relative humidity of less than saturation. In this 
case the relative humidity would be the pressure of the 
actual vapor, one pound, divided by the pressure which 
the vapor would have if it were saturated at 112 de- 
grees, viz., 1/1.35 = 74 per cent, humidity. 

If the argument has been closely followed it will 
now be evident that superheated vapor is the same 
thing as moist air with the air removed. The same ef- 
fects upon the material to be dried are produced in 
both cases so far as the vapor is concerned, but in the 
case of moist air, the effect of the air is added to that 
of the vapor. The same laws apply to the vapor 
whether the air is present or absent. The air conveys 
heat, but by its presence retards the diffusion of the 
vapor, and consequently retards the rate of evapo- 
ration. 

Relative Heating Capacities of Air and Vapor Sepa- 
rately. — To compare the relative heating capacities of 
dry air and of superheated vapor, the following de- 
ductions are made : The specific heat of water vapor at 
a pressure of one atmosphere is .475; that is to say, 
one pound of superheated steam in falling one degree 
Fahrenheit gives up 0.475 British thermal unit. To 
evaporate one pound of water at 212 degrees, there- 
fore, will require the heat given up by 966 (latent heat 



234 THE KILN DRYING OF LUMBER 

at 212 degrees) -=- .475 = 2034 pounds of steam falling 
one degree. At 212 degrees the volume per pound is 
26.78 cubic feet; therefore, 2034 X 26.78 = 54,470 cubic 
feet of superheated steam falling one degree are re- 
quired to evaporate one pound of water. The specific 
heat of dry air is 0.237 and the volume of one pound is 
16.93 (.05907 pound per cubic foot) at 212 degrees and 
atmospheric pressure. Therefore, to evaporate one 
pound of water at 212 degrees (966 B.t.u.) will re- 
quire the heat given up by 966 X ^£^ = 69,000 cubic 
feet of dry air falling one degree. It is thus seen that 
the heating capacity per unit of volume of superheated 
steam at atmospheric pressure is but little greater than 
that of dry air at the same temperature and pressure 
in the ratio of 69,000 to 54,470, or about 5 to 4. At 
temperatures above 212 degrees and the same pressure 
of one atmosphere, a greater volume is necessary to 
produce the same effect, since the gas and vapor expand 
with temperature, but the ratio of the heating capacity 
of superheated steam and dry air remains very nearly 
the same. The specific heat of vapor increases slightly 
at higher temperatures. Thus figuring in a similar 
manner it will be found that at 5 atmospheres pressure 
(59 pounds gauge) the heating ratio of equal volumes 
of steam and air is 1.42 to 1, and at one pound abso- 
lute pressure or a vacuum of 28 inches, it is 1.104 to 1. 
The volume of steam at 5 atmospheres pressure and 



CONSIDERATIONS AND CALCULATIONS 235 

306 degrees Fahrenheit in falling one degree necessary 
to evaporate one pound of water at this pressure and 
temperature is 10,336 cubic feet, and at a vacuum of 28 
inches at 102 degrees it is 829,433 cubic feet. 

Thus it is seen that there is but little advantage 
from the point of view of the volume of gas to be moved 
in the use of superheated steam over that of dry air. 

In this discussion a cubic foot of space has been 
used as the basis of the calculations. In analyzing the 
heat quantities in the drying operation, it will be easier 
to use one pound of dry air as a basis, with its accom- 
panying moisture, and follow it through its various 
stages. Its volume will therefore not remain fixed, but 
will change with every change in temperature and con- 
sequently the degree of saturation produced by a defi- 
nite amount of moisture accompanying it will depend 
upon the volume which it occupies. 

THEORETICAL AN^ALYSIS OF HEAT QUANTITIES 

For this purpose the simplest way will be to follow 
a pound of dry air through a drying cycle as a basis for 
computations. While in reality the vapor does not 
enter the air like water in a sponge, but occupies the 
same space whether air be present or not, we may, for 
convenience, conceive of a pound of air as containing a 
certain amount of vapor, which, in reality, means that 
the space occupied by a pound of dry air under given 
conditions contains a certain amount of vapor. 



236 THE KICN DRYING OF LUMBER 

Vapor and Air in Mixture. — As already explained, 
the total pressure always is the sum of the individual 
pressures of the air alone plus the vapor alone. Thus, 
we may speak of a pound of air as being wholly or par- 
tially saturated with vapor, meaning that it is the space 
occupied by the pound of air which is in this condition 
of vapor. If a pound of air said in this sense to be con- 
taining a given weight of vapor be heated a given 
amount under the same pressure of one atmosphere 
both air and vapor will expand the same amount, so 
that at the new temperature both will occupy the same 
amount of space again, which is greater than before, 
but the pound of air will still contain the same weight 
of vapor. It would be well to note, however, that the 
amount of vapor contained in a pound of air alone 
when it is saturated can not be used as the divisor in 
obtaining the relative humidity when compared to the 
amount of vapor actually contained in the pound of 
air alone, for the reason that when the air is saturated 
the pressure of the air alone will have been reduced 
corresponding to the increase in the vapor pressure 
(since the sum of the two makes up one atmosphere), 
so that for a pound of air a much greater space is re- 
quired and consequently an equivalently greater weight 
of vapor will be occupying this larger space. For 
relative humidity it is necessary to compare the weights 
of vapor which occupy the same amount of space when 



CONSIDERATIONS AND CALCULATIONS 237 

partially or wholly saturated, or, better still, to com- 
pare the vapor pressures, since no confusion is then 
likely to arise. 




Fig. 52. — Diagrammatic plan of Drying Cycle. 

Cycle in Drying Operation of One Pound of Air. — 
Let us follow the pound of dry air through its cycle of 
operation : Let the air enter the heater either from out- 
side or from the spray chamber at temperature t^, and 
let it contain d^ pounds of vapor. (See Fig. 52.) Both 



238 THE KILN DRYING OF LUMBER 

the air and the vapor are raised to the temperature t^ 
after passing through the heater, each pound of air still 
contains d^ pounds of moisture, since the vapor ex- 
pands the same amount as the air in heating from t^ 
to tz and both occupy the same volume, which is greater 
at the higher temperature, d^ and d^ are therefore the 
same so long as no vapor is added or subtracted during 
the heating from t^ to tz. In passing through the lum- 
ber, they become cooled to t^ and an additional amount 
of moisture w is added from the evaporation so that 
the pound of air at temperature ^3 now contains d^ = 
di-{- w pounds of moisture. Thence they either 
escape into the outer air as in a ventilating kiln or pass 
into the spray chamber where the heat added by the 
heater and the extra amount of moisture w is removed 
from the pound of air into the spray water, and it is 
returned at the initial temperature i^i saturated to re- 
peat the cycle. The changes in total pressure will be so 
slight that they may be neglected and the whole opera- 
tion be considered to take place at a uniform pressure 
of one atmosphere. Let r equal the specific heat of 
air at constant pressure and s that of superheated 
vapor. These will be taken as 0'.237 and 0.475 re- 
spectively. Then the quantity of heat imparted to the 
pound of air and its accompanying d^ pounds of vapor 
by the heater is (1), (0.237 + ^1 X 0.475) (t^ — t^) and 
the amount of heat given up in evaporating the water 



CONSIDERATIONS AND CAI^CULATIONS 239 

w is (2), (0.237 + d,X 0.475) (^2 — ^3). The amount 
of water evaporated is w={d^ — <?i). Now the heat 
required to evaporate the water w in continual opera- 
tion will be that required to raise it from its initial 
temperature to the evaporating point plus the latent 
heat of vaporization at this point ; also the heat neces- 
sary to raise the temperature of the wood alone the 
same amount. As the latter is small it will be neg- 
lected for simplicity. Suppose for a convenient ex- 
ample that the initial temperature of the outside air 
and of the wet wood be 32° Fahrenheit. Then the 
heat required is simply the total heat H of w pounds 
of vapor at the temperature t^ (nearly).* Hence (3), 
(0.237 + ^1 0.475) (t^ — t^) =wH = (d^ — d^) H or 
t^ — t^ H 



d^ — d^ 0.237 + d^ 0.475 

In this equation ^2 is a known quantity, being dependent 
upon the kind and condition of the material being dried. 
d^ is known, being the weight of moisture of the outside 
air per pound of dry air or the weight required to 
saturate one pound of air in the spray kiln at the tem- 
perature t^. H is known approximately (but not ex- 
actly, since its value varies with ^3 or more properly 
with the wet bulb temperature) and may at first be 
assumed for some temperature between t^ and ^1 and 
afterward be correctly assigned, t^ and d^ are the 

* Evaporation will actually take place at the temperature of the 
wet bulb if the air is not saturated, after which the vapor is superheated 

to tg. 



240 THE KILN DRYING OF LUMBER 

unknown quantities required. If the air is to be con- 
sidered saturated at t^, then 3^3 and d^ are dependent 
variables, their equation being that of the curve of 
saturation for water vapor. As the equation is com- 
plex their relative values can be more readily obtained 
from a table of saturated vapor and successive values 
substituted in equation (3) until the equation is ful- 
jfilled. Having thus determined ^3 approximately the 
correct value for R may be inserted and the more 
exact value of ^3 determined. This has been done by 
E. Hausbrand in ''Drying by Means of Air and 
Steam" ^ for different temperatures of t^ and t^^ as 
well as for different humidities and pressures. 

Eficiency of Operation. — ^With no air present, that 
is to say with water vapor alone, under a so-called 
''vacuum" or with "superheated steam" at pressures 
of one atmosphere or greater, all the heat may be 
utilized in evaporating the moisture, the leaving and 
entering temperatures being the same and the pressure 
constant. With air present, however, and the pressure 
constant it follows that if the entering air be saturated, 
the leaving air must be at a higher temperature in 
order that it may contain the additional vapor at the 
same pressure. Thus a greater amount of heat is 
required than that utilized in evaporation, in raising 

" Translation from the German by Wright. Puhlished by Scott, 
Greenwood & Sons, 1901, 



CONSIDERATIONS AND CALCULATIONS 241 

the temperature of the air leaving the lumber. There 
is another combination of conditions possible in which 
the temperature at exit may be the same or even less 
than that of the entering air or vapor. With air 
present this is only possible by decreasing the pressure 
below that of the entering saturated air. In this case, 
the heat supplied may be even less than the theoretical 
amount required for vaporization, and the theoretical 
efficiency as reckoned by temperatures is more than 
100 per cent., the advantage gained being at the expense 
of the heat energy in the departing air and vapor, being 
somewhat analogous to the case of the condenser in a 
steam engine. The gain in heat is from the fact that the 
entering air is at a higher temperature than that leav- 
ing. If the entering air is not saturated a similar 
condition is also possible, since some evaporation may 
take place without necessitating a higher temperature 
of the leaving air. 

From the foregoing it might be concluded that theo- 
retically the use of a vacuum or of superheated steam 
would be the most economical way in which to dry ma- 
terials. In the practical working, however, the vacuum 
has certain disadvantages, as explained heretofore, the 
chief one being the greater volume of vapor required 
and the difficulty of producing a uniform circulation 
of vapor at high attenuation. The other drawback is 
the expense of the apparatus and difficulty of opera- 

16 



242 



THE KILN DRYING OF LUMBER 



tion at pressures other than atmospheric. With super- 
heated steam the temperature is too high for most 
woods. 

Concrete Example of Heat Quantities. — To illus- 
trate the relations of these quantities under the various 
conditions, let us take a concrete example where the 
initial temperature of the air is 32° and the air is 
saturated both at the entrance and upon leaving. This 
is heated to 158° and then passed through the material 
to be dried. The volume of the gas required at the 
temperature of 158° and the theoretically least possible 
expenditure of heat required to evaporate one pound 
of water from an initial temperature of 59° Fahren- 
heit at various pressures are given below. 



Absolute pressures 


Volume 

in 
cu. ft. 


TotaJ 

heat 
required, 
B. T. U. 


IJ^ atmospheres 


695 

876 

1247 

2121 

16,821 


2010 


1 atmosphere =760 mm. of mercury 


1692 


500 mm. of mercury, partial vacuum 


1578 


250 mm. of mercury, partial vacuum 


1346 


Using steam alone superheated from 140° to 158° at 
pressure of 148 mm. of mercury, corresponding to 
saturated conditions at 140° Fahrenheit 


1125 







The minimum theoretical expenditure of heat as 
here calculated has no direct bearing as to the efficiency 
of any method of drying lumber, however, since the 
physical requirements of the lumber may, and generally 
do, demand conditions totally incompatible with the 



CONSIDERATIONS AND CALCULATIONS 243 

highest theoretical heat efficiency. They apply directly 
only to the evaporation of a free body of water, irre- 
spective of length of time required and with no radia- 
tion losses. The calculations are useful, however, in 
showing the limiting values of the efficiency which it is 
possible to attain under the conditions which have 
otherwise been found most suitable for drying the 
lumber in question. 

It is instructive to know the highest possible theo- 
retical efficiency in evaporating a pound of water under 
given conditions, considering no losses in radiation or 
otherwise. For this purpose the following table (IX) 
has been worked out, assuming the water to start with 
an initial temperature of 59° Fahrenheit and to evap- 
orate at the temperature i^, which is the temperature 
of the leaving air. The efficiency here expressed is 
the ratio of the total heat of water vapor at t^ above 
59° divided by the least possible expenditure of heat 
necessary to evaporate it under the assumed conditions 
of the entering and leaving air at atmospheric pressure. 
When the temperature t^ of the entering air approaches 
that of the heated air ifg, that is, when a high humidity 
is used, the calculations become very uncertain, since 
the quantity of air called for under the assumed con- 
ditions approaches infinity, while the temperature dif- 
ferences between t^ and t^ become infinitesimal. 

The minimum volume of air required to evaporate 



244 THE KILN DRYINQ OF LUMBER 

one pound of water is also given in table (Table IX). 
Generalisation. — A study of the theoretical heat 
relations, as shown by Hausbrand's tables, makes pos- 
sible the following generalizations : 

1. With 1^2 constant and entering air saturated, the 
expenditure of heat is less the higher the temperature 
ti of the entering air. 

2. With ^1 constant, the expenditure of heat is less, 
the higher the temperature tz to which the air is heated. 

3. Other things being the same, the heat expenditure 
increases rapidly with reduction in humidity of the 
emergent air. 

4. Other things being the same, the heat expendi- 
ture is less, the lower the humidity of the entering air. 

5. Other things being the same, the expenditure of 
heat increases with increase of pressure. 

6. With water vapor in the absence of air the theo- 
retical efficiency becomes one hundred per cent. 

In regard to the weights and volumes of air re- 
quired, the following observations are obtained : enter- 
ing air saturated ; 

With ^2 constant, both the weights and volumes of 
air required to evaporate one pound of water increase 
with increase of the initial temperature ^i of the enter- 
ing air. 

With f 1 constant, both weights and volumes decrease 
with increased temperature t^ of the heated air. 



CONSIDERATIONS AND CALCULATIONS 



245 



Table IX. — Maximum Possible Theobetical Heat Efficiency op 
Evaporation Under Given Conditions (ti, Tj, Hi, H3) at Atmospheric 
Pressure (760 mm.) 



Entering air 



After heating 



b> 



Leaving air 



h. 



Heat con- 
sumed to 
evaporate 
1 lb. of 
water, 
B. t. u. 
from ini- 
tial tem- . 
perature of 
59° F. 



Total heat 

of 1 lb. 

of vapor at 

tj above 

initial 
tempera- 
ture of 59° 
F. 



Mini- 
mum 

volume 
of air 

required 



Effici- 
ency 

H-^Q 



E 



K 



De- 

•grees 

32 
59 

32 

59 
86 

32 
59 
86 

32 

32 
86 

32 



^32 
59 
86 

32 

86 

140 

32 

86 
176 



Per 
cent. 

100 
100 

100 
100 
100 

100 
100 
100 

100 

100 
100 

■100 

;ioo 

100 
100 
100 

100 
100 
100 

100 
100 
100 



De- 
grees 

95 
95 

158 
158 



Per 
cent. 

11 
31 



2 
6 
158 13 



212 
212 
212 

95 

158 
158 

212 
212 

95 
95 
95 

158 
158 
158 

212 
212 
212 



0+ 

2 

4 

11 

2 
13 

0+ 

4 

11 
31 
74 

2 
13 
63 

0+ 
4 

47 



De- 
grees 

65 

76 

84 

92 

107 

97 
103 
114 

84 

110 
141 

126 
146 

60 
70 



79 
99.5 
140.9 

90 

106 
176.5 



Per 

cent. 

75 
75 

75 
75 
75 

75 
75 
75 

25 

25 
25 

25 
25 

100 
100 
100 

100 
100 
100 

100 
100 
100 



B. t. u. 

2353 
2100 

1911 
1715 
1556 

1758 
1572 
1422 

6136 

2972 
4869 

2352 
2166 

1974 
1679 
1476 

1692 
1390 
1119 

1582 
1350 
1130 



B. t. u. 

1074 
1078 

1080 
1082 
1087 

1084 
1086 
1089 

1080 

1088 
1098 

1093 
1099 

1073 
1076 
1081 

1079 
1085 
1098 

1082 
1087 
1108 



cu. ft, 

2163 
3426 

993 
1126 
1402 

694 
731 
796 

6738 

1495 
4385 

930 
1206 

1836 
2733 
9725 

876 
1329 
3879 

625 

721 

2002 



.457 
.514 

.665 
.631 
.698 

.617 
.690 
.767 

.176 

.366 
.225 

.457 
.607 

.544 
.641 
.733 

.636 

.781 
.981 

.684 
.804 
.972 



In water vapor alone : 



140 


100 


158 


63 


140 


100 


1097 


1097 


16418 


1.00 


212 


100 


230 


71 


212 


100 


1119 


1119 


3657 


1.00 


212 


100 

- 


320 


16 


212 


100 


1121 


1121 


664 


1.00 



246 THE KILN DRYING OF LUMBER 

With the emergent air only partially saturated, the 
weights and volumes increase with decrease of relative 
humidity in the emergent air. 

Conclusion as to Efficiency of Operation. — From 
this analysis of heat equations the following conclusions 
as regards the efficiency of the drying may be drawn : 

1. The air should be heated to the highest tempera- 
ture compatible with the nature of material to be dried. 

2. The air upon leaving the apparatus should be as 
near saturation as practicable. 

3. The temperature of the entering air should be as 
high as possible. 

Application of Analysis to Water Spray or Con- 
denser Kiln. — The above deductions apply to any form 
of moist air kiln. The following have more especially 
to do with our water spray humidity regulated kiln. 

The amount cf heat absorbed by the spray water 
and the condensed moisture is the difference between 
the total heat in the saturated air as it leaves the lumber 
at ^3 and the total heat in the air at t^. It is, in fact, 
the amount of heat given up by the coils, since the air 
is brought back to its initial state in the cycle and the 
water evaporated from the wood is added to the spray 
water. Hence the amount of heat removed in water 
at a temperature t^ is (4), G (^2 — ^i) X (^ + 5^i), 
when G is the weight of dry air in the mixture required 
to evaporate one pound of water, r is the specific heat 



CONSIDERATIONS AND CALCULATIONS 247 

of dry air and s that of tlie water vapor at constant pres- 
sure. Of this the amount G {t^ — ti) {r -^ sd^^) repre- 
sents the loss not accounted for in the latent heat of the 
pound of water which has been evaporated and is taken 
up by the spray water. The maximum possible thermal 
efficiency is therefore 

(5) ^'^~^^l on the condition that just enough air 
is circulating to give up all its available heat to the 
evaporation of the water so that it leaves the lumber 
in a saturated condition. From equation (2) and (3) 
the v^l'je of t^ is determined for any given values of 
ti and ^2- These values may be most readily obtained 
from the tables given by Hausbrand, referred to above. 
ti and 1^2 ^^^ arbitrary values determined entirely by 
the physical conditions of the material to be dried. 

In actual operation, however, the efficiency will be 
much less than this maximum, since the air leaving will 
not be saturated and a much larger quantity of air 
will need to pass through the material than theminimum 
indicated by this equation. If no evaporation takes 
place all the heat will be used in heating and cooling 
the circulating medium. The total heat used per pound 
of air will then be (^2 — ^1) (^ + S(^i) and this will go 
simply to heating the spray water. 

Comparison of Efficiency. — Comparing the efficiency 
of this kiln with that of the ventilating type, it will be 
seen that under identical running conditions its effi- 



248 THE KILN DRYING OF LUMBER 

ciency is much greater because the initial temperature 
^1 is very much higher. Let the temperature of the out- 
side air be 32°, so that the water has to be raised from 
32° to the temperature of evaporation and then evapo- 
rated. Let the air leaving the lumber be three-quarters 
saturated, 75 per cent, humidity. Also let f i = 113° and 
1^2=^140°, giving a relative humidity of 48 per cent. 
Then d^^ for one pound of saturated air at 113° is 0.0653 
pound. Substituting these values in equation (3) to 
find ts and after several trial balances of the values of 
ds at 75 per cent, humidity, these values are found to 
be ^3=^125° and <^3 = 0.06889. Since w^=d^ — d^, 
the number of pounds of air required to evaporate one 
pound of water is G = i = -^ = 279, which 

^ W 0.3- di 

contains 279 X 0.0653 = 18.2 pounds of vapor. The 
pressure of the saturated vapor alone at 113 degrees 
is 71.4 mm. of mercury ; hence, that of the air alone is 
760 — 71.4=^688.6 mm. of mercury. The volume 
occupied by one pound of dry air at 113° and a pressure 
of 688.6 mm. of mercury is 16 cubic feet (more exactly 
15.921), which must be the same as that occupied by 
the 0.0654 pound of vapor present in the pound of 
air. As 279 pounds of air are required, with its in- 
herent 18.2 pounds of vapor, the volume of air or com- 
bined air and vapor is 15.921 X 279 = 4442 cubic feet 
at 113°. At 125° this will occupy 4535 cubic feet. 
The total heat consumed is 279 (0.237 + 0.0653 



CONSIDERATIONS AND CALCULATIONS 249 

X 0.475) X (140—113) = 2019 B.t.u., of which the use- 
ful work has been the total latent heat of one pound of 
vapor above 32° evaporated at 116° (the wet-bulb tem- 
perature) and superheated to 125° = 1122 B.t.u. This 
should be the same as the heat given out by the air and 
superheated vapor in cooling from 140° to 125°, 279 
(0.237 + 0.0653 X 0.475) (140 — 125) =^ 1122. The 

thermal efficiency is ^^^ = 1%=^ = 55.6 

ti — h 140 — llo 

per cent. Also ^^ = 55.6 per cent.^ 

Compare this now first with a ventilating kiln in 
which the air enters saturated at 32°, is heated to 140° 

® Another way of arriving at this result is to compare the total 
heats J thus, in the vapor at 125° and 75 per cent, saturation: 

Total heat in the air alone at 125° = 279 X .237 

(125—32) 6,149 

Total heat in saturate vapor at the dewpoint of 115° 
(75 per cent, humidity at 125°) = 279 X .06889 
X 1117 21,491 

Superheating this vapor from its dewpoint of 115° 

to 125° = 279 X. 06889 X .475 X 10 pi 

Total at 125 degrees 27,731 

At the initial stage 113 degrees: 

Total heat in air = 279 X .237 (113—32 ) 5,356 

Total heat in saturate vapor at 113 degrees = 279 X 

.0653 X 1116.4 20,339 

Total heat at 113 degrees 25,695 

The difference, 27,731 — 25,695 = 2,036 B.t.u., is the heat added 
to the air. This should be the same as before, namely, 2,019, the dif- 
ference being in inaccuracy of the constants used. 



250 THE KILN DRYING OF LUMBER 

and leaves at 75 per cent, humidity, escaping to the 
outer air. We then have : 

*i = 32 degrees, di = .00387 pound per pound of air 

*2 = 140 degrees 

tg = calculated = 80.2, and d^ at 75 per cent, humidity = .01692 

The quantity of air required to evaporate one pound 
of water is : 

_ .01692 - .00387 ^^ ^ 

G=^ J = 76.6 pounds 

This air contains 76.6 X 0.00387 =:- 0.296 pound of 
vapor. The total heat consumed is : 

76.6 ( .237 + .00387 X .475 ) ( 140—32 )= 1,969 B. t. u. 

The thermal efficiency is ^^~f^ = 55.6 per cent., 

J.4U — 0:Z 

which happens to be the same as in our kiln, but exam- 
ination will show at once that the two cases are not 
analogous. In our kiln the humidity after heating to 
140° was 48 per cent.; in the other kiln it is only 3 
per cent., an extremely low amount. 

In order to compare the two cases correctly the 
condition of the air entering the lumber should be the 
same in both cases ; namely, it is necessary to raise the 
humidity in the ventilating kiln from 3 per cent, to 48 
per cent. This can be done by allowing live steam to 
escape into the heated air sufficient to saturate it at 
113°, as this is the dewpoint for 48 per cent, humidity. 
Now if one pound of dry air saturated at 32° be heated 
to 113°, it will still contain its original weight of vapor ; 
namely, 0.00387 pound, but to saturate a pound of air 
at 113° requires 0.0653 pound of vapor; consequently, 



CONSIDERATIONS AND CAI.CULATIONS 251 

the difference between this and 0.00387 or 0.06143 
pound of vapor must be added for each pound of air at 
113° in order to make the two cases comparable; they 
are then exactly alike, and we shall have for our kiln, 
to recapitulate, as before: 

*i = 1 1 3 ° saturated 
4 1=140° humidity 48 per cent. 
*2=125° humidity 75 per cent. 

Number of pounds of air required to evaporate one pound 
of water at 115° from initial temperature of 32°=: 279 
Total heat required = 2,019 B.t.u. 
Heat lost,' 2,019 — 1.122 = 0,897 B.t.u. 

In the ventilating kiln, on the other hand, we shall have 
by comparison: ti=^S2° saturated; ^2 = 140° at 3 
per cent, humidity; ^3 = 125°, humidity 75 per cent; 
J12 = heat in vapor added to raise the humidity to 
saturation at 113° ; 0.0614 pound is required per pound 
of air. The total heat in saturate vapor at 113° above 
32° = 1117 B.t.u. per pound; 1117 X 0.0614 = 68.58 
B.t.u. required per pound of air. There are 279 pounds 
of dry air required as in the other case. 68.58 X 279 = 
19,134 B.t.u., which must be added as vapor. 

K2 = heat required to raise temperature of the air 
and vapor from 32° to 113° = 279 (0.237 + 0.00387 X 
0.475) (113° — 32°) = 5396 B.t.u. 

■^ In the spray kiln this is not in reality lost, since part is utilized 
in producing the circulation and all the remainder is recovered in the 
spray water. It is simply a transfer of heat from lumber to spray 
water. 



252 THE KILN DRYING OF LUMBER 

Therefore, in this case, the total heat which must 
be given to the air to evaporate one pound of water is : 

Heat given by coils to raise the air from 32° 
to 113° = 5,396 B. t. u. 

Heat given by coils to raise saturate air from 

113° to 140° as before= 2,019 B. t. u. 

Heat supplied in vapor = 19,134 B. t. u. 

Total beat required^ 26,549 B. t. u. 

Heat lost (provided it all escapes to the air) 26,549 — 1,122 = 
25,427 B. t. u. 

Compared to the loss in our kiln as just shown of only 
897 B.t.u., this would be enormous. It would mean an 

efficiency of only ^rZo? ""^ ^-^^ P®^ cent. The assump- 
tion, however, that it all escapes to the outside air is 
not carried out in practice in the moist air kilns, but 
instead, a large proportion of this is returned by in- 
ternal circulation and only a small amount escapes 
into the air. It is not possible in the latter case to 
calculate the theoretical efficiency, since there is no 
means of knowing what portion of the heat is returned 
in the recirculation within the kiln. The analysis is in- 
structive, however, in showing what enormous heat 
losses are possible in a ventilating kiln, and the par- 
ticular object is in showing that in no case can the 
efficiency of the ventilating equal that of our kiln when 
operating under identical conditions within the drying 
chamber. 

Tendency of Air to Descend Through a Pile of Wet 



CONSIDERATIONS AND CALCULATIONS 



253 



Lumber. — The metliod of piling wliich gives the best 
results is such that the heated air passes through the 
pile in a somewhat downward direction; the natural 
tendency of the cooled air to descend is thus taken 
advantage of in assisting the circulation in the kiln. 
This is especially important when cold or green lumber 
is first introduced into the kiln. But even when the 
lumber has become warmed the cooling due to the 
evaporation increases the density of the mixture of 
the air and vapor. In Table X it is shown analytically 



Table X. — Incbease in Density of Mixture of Aie and Vapor Pro- 
duced BY THE Spontaneous Cooling of the Mixture from 
THE Evaporation of Moisture as it Passes 
Through the Lumber. 

The weights are given in gramm/es per cubic centimeter of the mixture. 
The independent variables which may be assumed at choice are ( 1 ) the 
temperature of the entering air *i; (2) the relative humidity of the 
entering air hi; (3) the temperature to which the air is heated before it 
enters the lumber #2; and (4) the degree of saturation of the air leaving 
the lumber, h^. From these, J^, U, and the volumes and weights of the 
air and vapor are determined. 



Entering air 


After heating before 
entering lumber 


Leaving lumber 


Weight of 1 
ture in 


c.c. of mix- 
grams 


ti 


hi 


ti 


hi 


Dew- 
point 


U 


h3 


Entering 
at tihi 


Leaving at 
iihi 


Degrees 


Per cent 


Degrees 


Per cent 


Degrees 


Degrees 


Per cent 






32 


100 


158 


2 


32 


78.8 


100 


.0010264 


.0011658 


32 


100 


158 


2 


32 


110.5 


25 


.0010264 


.0011057 


86 


100 


158 


13 


86 


99.5 


100 


.0010126 


.0011094 


86 


100 


158 


13 


86 


140.5 


25 


.0010126 


.0010394 


140 


100 


158 


63 


140 


140.9 


100 


.0009525 


.0009779 


140 


100 


158 


63 


140 


151.7 


75 


.0009525 


.0010154 


86 


100 


212 


4 


86 


105.8 


100 


.0009310 


.0010915 


86 


100 


212 


4 


86 


146.3 


25 


.0009310 


.0010255 


176 


100 


212 


47 


176 


176.5 


100 


.0007820 


.0008221 



254 THE KILN DRTING^ OF LUMBER 

that the spontaneous cooling of the mixture produced 
by the evaporation alone increases its density. 

This fact is of great significance, and the method 
adopted of piling the lumber in the new kiln takes 
advantage of this principle. 

Method Used in Calculating Table IX. — 1. The tem- 
perature, t^, of the air leaving the lumber is determined 
first, as for Table I. The dewpoint must also be de- 
termined in order to determine the vapor pressure e. 

2. The following equation gives the value of the 
density (grammes per cubic centimeter) of the mixture 
of air and vapor: 

B- 0.378 e ,, .00129305 * 

" = STTT^ X 



760 1 + .003670 t 

J? = total barometric pressure in millimeters of mercury. 

e = pressure of the vapor in the mixture. 

t = temperature Centigrade of the mixture. 

.00129305 is the weight in grammes of 1 c.c. of dry air at 
0° C, pressure 760 mm. under gravity at 45° latitude 
and sea level. The figure .003670 is the coefficient of 
thermal expansion of air at 760 mm. 

The first fractional expression may be explained 
as follows : 

Let di = density of dry air at B-e m.m pressure. 
dv = density of vapor at e m.m. pressure 
Then d = di + dv. The air pressure alone is B—e 

and di = do -=^ 

dr = .622 XdoX~Q 
where 0.622 is the density of vapor compared to air at 760 pressure. 

* See Smithsonian Meteorological Tables, Table 83 to 86. 



CONSIDERATIONS AND CALCULATIONS 255 

Whence . = ..{B-e +:6-X,|= ,.|B_^e| 

Knowing the values ^2 and t^ and the vapor pressures 
at these two points (pressures at the dewpoints), the 
values of dz and d^ are obtained from the above 
equation. 

It will be noted that in every case chosen in Table 
III, the density increases, due to evaporation, hence, 
the tendency of the air is to descend as it passes through 
the pile of lumber. 



CHAPTEE XI 

Effect of Differen^t Methods of Hrying Upon the 
Strength and the Hygroscopioity op Wood 

A RESEARCH was conducted from June, 1905, until 
October, 1908, at the Forest Service Timber Test Lab- 
oratory at Yale University, at that time being con- 
ducted in cooperation with the university, to determine 
the effect of subjecting both green and air dry wood 
to various temperatures and humidities of the sur- 
rounding medium for different lengths of time. Com- 
parative tests were then made in both static bending 
and static compression on the untreated and treated 
specimens. The plan followed was to cut the material 
into carefully selected series of matched specimens. 
Seven series for each kind of test and each treatment 
were prepared from fresh green wood and the values 
of the corresponding sets averaged. This gave an 
average of seven tests for each individual determina- 
tion. Each series was composed of six '' sets " of 
Bpecimens, numbered from 1 to 7 for the bending and 
from 11 to 17 for the compression, coordinate through- 
out each of the seven series, each set receiving a sep- 
arate treatment. Sets 1 and 3 were used untreated 
as standards for comparison for the green and the air 

256 



EFFECT OF DIFFERENT METHODS 257 

dry, respectively. The treatments of each "set" 
throughout all of the experiments can best be shown in 
tabulated form, thus : 

Set 1 — (standard) soaked green. 

Set 2 — soaked, subjected to process, resoaked, and 
compared with Set 1. 

Set 3 — (standard) air dried about two years. 

Set 4 — air dried, subjected to process, air dried to- 
gether with Set 3 about one year after treatment. 

Set 5 — soaked, subjected to process, air dried about 
one year together with Sets 3 and 4. 

Set 6 — soaked, subjected to process, generally tested 
directly to show the direct effect of the treatment. 

The bending tests were made with a span of 26 
inches, the specimens being 2 inches high and II/2 inches 
wide, and the compression specimens were li/^ X 1% X 
534 inches, except when cut to smaller size on account 
of shrinkage. All specimens were treated in the rough 
and cut to exact size at time of test. 

The moisture determinations were made by the 
usual method of cutting an inch disc across the block 
at the region of failure and drying in an oven at 98° 
Centigrade. The loss in weight times 100 divided by 
the dry weight is the moisture per cent. used. Another 
check disc was cut from some other portion of the stick 
in each case and the moisture determined in the same 
manner. 
17 



258 THE KILN DBYING OF LUMBER 

Three species of wood were used : White ash from 
Connecticut, loblolly pine from Charleston, S. C, and 
red oak from Connecticut. More kinds of treatment 
were given the pine and oak than the ash. Each treat- 
ment is designated by the term *' process" to avoid 
confusion with the common use of the word "treat- 
ment" as referring to impregnation with creosote. The 
processes included dry air, moist air, saturated steam, 
superheated steam, and temperatures from 145° to 
331° Fahrenheit. The processes are given in Tables 
VII, VIII, and IX. 

The research embodies over two thousand mechan- 
ical tests on carefully selected and matched specimens, 
and thirty-seven processes, and covered a period of 
over three years. The full report of the research is 
filed at the Madison Laboratory of the Forest Service 
(Project L-16). The high temperature and high pres- 
sure treatments were made in a small iron cylinder 
twelve by forty-four inches in size, which was con- 
structed especially for the purpose. The kiln drying 
treatments at 145° F. were made in a masonry-lined 
room heated by horizontal layers of steam pipes, and 
the higher temperature drying in a water- jacketed 
copper oven. 

In the tables a minus sign indicates a reduction of 
strength as compared to the respective standards and 
a plus sign shows an increase. 



EFFECT OF DIFFERENT METHODS 



259 



The results are given in comparative terms to tlie 
standards, and the values of the modulus of rupture 
and the crushing strength only are used in the com- 
parisons for brevity. These tables have been greatly 
abridged from original report, and are intended to 
show only the general conclusions. In order to make 
the comparisons as clear as possible, the values for 
Sets 4 and 5 have been corrected for moisture so that 
they correspond to the same moisture condition as 
the standards, Set 3. For this purpose the moisture 
strength curves in the files of the Forest Service were 
used. The curve for black oak has been used as ap- 
plying to red oak and the compression strength curve 
has been used in reducing the values for the modulus 
of rupture. This may account for the apparent in- 
crease in strength shown in the air drying compression 
tests in the red oak table, Processes A to D. 

Table XI. — Weakening Effect of Various Processes of Drying 
ON THE Strength of White Ash 





Reduction in strength in per cent, of normal 


Prooeas 


Compression sets 


Bending sets 




2 

soaked 


4 
air-dry 


5 

air-dry 


2 
soaked 


4 
air-dry 


5 

air-dry 


A. Dry air 145" F., 25 days 

G. Saturate steam 212=" F., 1 hour. . . 
H. Saturate steam 212'' F., 4 hours. . . 
L. Saturate steam 90 lbs. 331° F., 


-23.4 
-12.5 
-10.3 

-44.7 

-24.8 
-15.2 

-11.9 






-27 

- 4 














-3 


- 9.5 

'-' i'.s 

-16.0 
+ 0.2 

-22.8 




"6 

-3 



+5 
±2 


M. Saturate steam 90 lbs. 331° F., 
5 minutes 






N. Exhaust steam 145°, 19 days 

R. Superheated steam 331° F., at 
atmospheric pressure, 6 hours . . 



+ Increase in strength. 
— Decrease in strength. 



260 



THE KILN DRYING OF LUMBER 



Results of Treatments. — Two general effects were 
noted: (1) That all processes used in this research re- 
duced the strength of the wood, when it was resoaked 
and compared with the untreated green-soaked stand- 
ard; and (2) that the hygroscopicity was reduced by 
most of the treatments and the color darkened, par- 
ticularly in the higher temperature treatments. This 
means a reduction in the subsequent shrinkage and 
swelling or the ''working" of the wood, as it is called. 

Table XII. — Weakening Effect of Various Processes of Drying 
ON THE Strength of LobloLlLT Pine 



Treatment 



A. Dry air 145° F., 26 days 

B. Dry air 170° (bending tests), 8 days 

C. Dry air 200°, 3 days 

D. Dry air 250°, 6 hours 

Note. — B, C, and D were all 
dried under A previously. 

F. Saturate steam 212°, 8 hours 

G. Saturate steam 212°, 1 hour 

H. Saturate steam 212°, 4 hours 

K. Saturate steam, 90 lbs., 331° F., 

15 minutes 

L. Saturate steam, 90 lbs., 331° F., 
4 hours 

M. Saturate steam, 30 lbs., 274° F., 
4 hours 

N. Exhaust steam 145°, 22 days 

P. Exhaust steam 145° F., 22 days, 
followed by exhaust steam 170° 
2 days (previously with N) . . . . 

R. Superheated steam atmos. pres- 
sure 274° F., 4 hours 

S. Superheated steam, 30 lbs. pres- 
sure, 298° F., 4 hours 



Reduction in strength in per cent, of normal — 



Compression sets 



2 
soaked 



-13.1 
-20.0 



-20.0 
-24.8 
-19.7 

-27.3 

-36.6 

-22.1 
-24.5 

-21.7 
-18.0 
-20.8 



4 
air-dry 







- 9 

-42 

-11 


- 2 
-1-0 



6 

air-dry 



Bending sets 



2 
soaked 



-15.4 
-14.4 
-25.3 



-24.3 
-18.3 



— 9.0 



4 

air-dry 



-10 




5 
air-diy 



* The values :f or sets 5 have not been carried through the lengthy operation necessary 
for corrections for moisture and variations from normal, and are therefore omitted from 
this table. When compared directly with sets 4, however, they appear weaker, with the 
exceptions of processes A, C, and N in compression and process A in bending. This 
lesser strength of sets 5, shown especially in the bending tests, may be due to checking 
of the wet wood (set 5 being soaked before processing) which might weaken th« beams 
more than the compression pieces. 



EFFECT OF DIFFERENT METHODS 



261 



It will be noted that the oak is injured much more 
than the ash or pine. No significant injury in the sub- 
sequently air 'dried condition in the case of loblolly 

Table XIII. — Weakening Effect op Various Processes of Drying 
ON THE Strength of Red Oak 





Reduction in strength in per cent, of normal — 


Process 


Compression sets 


Bending sets 




2 

soaked 


4 
air-dry 


5 

air-dry 


2 

soaked 


4 
air-dry 


5 
air-dry 


A. Dry air 145° F., 30 days 


-7.9* 
-7.3 
-3.3 
-9.5 

- 9.0 
+ 13.8t 

- 1.5 

-44.5 
-39.9 

-47.0 

- 8.3 

-10.7 
-49.8 
-36.2 


-t-16.3 
-t- 5.5 
-1- 3.3 
+ 2.8 

-12.5 

- 4.4 

- 2.4 

-49.1 

-61.3 

-47.7 
+ 9.5 

+ 1.1 
-60.0 

-58.2 


- 5.7 
+ 1.1 

-13.7 
-24.9 


-7.2* 

- 4.9 

- 4.4 

-' 3.9 

-24.8 

- 4.1 

-31.1 


-14.8 
-18.7 
- 9.8 

■-ii".3 

-33.4 
-10.8 

-58.7 




B. Dry air 170^ F., 8 days 

C. Dry air 200" F., 7 days 




D, Dry air 273-302°, 4M hours 

Note.— B, C, and D were each 
dried in Process A at first. 

F. Saturate steam 212° F., 8 hours. . 

G. Saturate steam 212° F., 1 hour. . . 
H. Saturate steam 212° F., 4 hours. . 
K. Saturate steam 90 lbs. 331° F., 

6 minutes but 1 hour in all heat- 
ing and cooling 


-' 7.8 


L. Saturate steam 90 lbs. 331° F., 

3)^ hours but 4J^ hours in all. . 

M. Saturate steam 30 lbs. 274° F., 


- 6.1 


N. Exhaust steam 145° F., 21 days. . 
P. Exhaust steam 170° F., 2 days 

started with Process N 

R. Superheated steam 10 lbs at 274° 

F., 4 hours 


-18.9 


S. Superheated steam 30 lbs. 298°, F. 













* Corrected for temperature difference o± the wet wood; 10.5 per cent, reduction 
in strength for each 25° F. increase in temperature. Based on tests given in the original 
report. 

tAnomalous result, can not be explained, all factors and tests carefully revised show 
no discrepancies. This value has also been corrected for temperature. 

pine is shown in treatments at temperatures of 212° F. 
or less in the moist air or steam, or up to 250° in dry 
air. Thirty pounds steam for four hours, however, 
shows a decided weakening. In the case of red oak no 
significant injury is evident in the subsequent air-dry 
condition in dry air up to 273° or 300° F. for four and 



262 THE KILN DRYING OF LUMBEE 

a half hours, but saturate steam at 212° for four hours 
or longer shows a decided loss in strength. Super- 
heated steam at 274° F. shows very great loss in the 
case of the oak, both in the subsequently air-dry and in 
the soaked conditions, less loss in the ash, and no loss 
in the subsequently air-dry condition in the case of 
the loblolly pine. A study of the tables will reveal 
other important comparisons. 

It is probable that the toughness of the wood was 
in many instances reduced when the static strength 
values showed no appreciable falling off. A number 
of impact tests made at Purdue University, Lafayette, 
Ind., on analogous material selected from this same 
lot, showed that the wood which had been subjected 
to all of the higher temperature treatments was more 
brittle than normal wood. 

In Table XIV the reduction in hygroscopicity pro- 
duced by the various processes is shown by the actual 
reduction of moisture content in the air-dry condition 
as compared with the air-dry standards. In this table 
a negative sign shows an increase in moisture content. 

Results Applied to Commercial Treatment. — ^In ap- 
plying these results to commercial treatments of large 
blocks of wood, the relative sizes should always be 
taken into consideration, since in larger sizes the ma- 
terial will not become heated through in the same 
length of time as with the small specimens used in 



EFFECT OF DIFFERENT METHODS 



263 



Table XIV. 



-Change in Htgroscopicitt Produced by the Various 
Processes 
white ash 





Compression 




Bending 


Process 


Moisture 
per cent. 


Reduction in moisture 
compared to set 3 in 
per cent, of dry wood 


Moisture 
per cent. 


Reduction in moisture 

' compared to set 3 in 

per cent, of dry wood 




Set 3 


Set 4 


Sets 


Set 3 


Set 4 


Set 5 


A 
G 
H 
L 

M 

N 
R 


14.9 
14.5 
14.0 
15.3 
15.3 
15.8 
15.2 


1.1 

0.8 
0.8 
4.8 
3.1 
1.7 
3.8 


0.1 
0.3 
0.4 
2.9 
-1.2 
1.3 
(1.5) 


15.0 
i5.2 

ii'.s 

15.4 
15.9 


2.0 

ols 
i'.s 

1.4 
6.8 


-0.2 

' 0.5 

-0.4 
0.9 



See Table I 



LOBLOLLY PINE 



A 


12.7 


0.8 


0.9 


13.4 


1.1 


1.2 


B 








14.3 


2.1 


1.8 


C 


i3.5 


i.3 


i.5 


15.4 


1.5 


1.5 


D 


13.9 


3.0 


1.8 








F 


12.4 


-0.4 


-0.6 








G 


13.7 


+0.4 


-0.6 








H 


13.8 





-0.2 


14.0 


0.2 


-0.2 


K 


13.2 


1.8 


0.5 








L 


13.7 


3.6 


3.5 








M 


14.3 


1.3 


1.0 


13.3 


1.6 


1.4 


N 


13.4 


1.3 


1.5 


14.4 


1.8 


2.0 


P 


14.1 


1.2 


1.2 


. • . 






R 


13.3 


1.1 


-0.1 


14.6 


2.1 


-0.1 


S 


15.4 


1.7 


0.8 








See Table 


II 













A 


12.4 


4.8 


2.8 


12.9 


2.5 


2.7 


B 


12.4 


1.8 


1.8 


13.3 


3.8 


3.5 


C 


12.0 


2.6 


2.5 


14.2 


3.2 


3.1 


D 


12.6 


3.7 


2.7 








F 


12.0 


1.0 


0.2 








G 


13.3 


0.3 


0.3 








H 


13.3 


0.4 




14.4 


1.0 


0.5 


K 


13.3 


3.2 


i.6 








L 


12.5 


2.8 


-0.5 








M 


12.9 


1.9 


1.8 


13.6 


2.5 


1.6 


N 


13.2 


2.0 


1.8 


15.1 


3.9 


0.5 


P 


13.2 


2.3 


1.4 








R 


13.4 


0.3 




13.7 


3.6 


1.3 


S 


13.5 


2.3 


i.3 








See Table 


III 













Not«. — Negative sign eignifies an increase in moisture. 



264 



THE KILN DRYING OF LUMBER 



this research. The interior of large blocks, such as 
railway ties, will also generally contain considerable 
moisture, even if treated in dry air or hot oil, which 
will change the conditions. The result on a large block 
will vary from the surface to the center, and the con- 
dition as a whole will be a combination of the effects 
produced on the several layers from the surface to the 
center. Nevertheless, these results will be true of 
each portion of the wood which becomes subjected to 
the same condition as those given in this research. 



t 

CHAPTER XII 

Insteuments Useful in Dry Kiln Work and Methods 
OF Testing Wood 

(1) Temperature. — The determination of tlie true 
temperatures in different portions of the dry kiln is 
of the utmost importance. It is, however, a difficult 
matter to do. An accurate thermometer does not neces- 
sarily register the temperature correctly. It depends 
upon how it is placed. In fact, it is less important 
to have the thermometer exact than it is to know how 
to place it so as to register the temperature which it is 
desired to determine. It is not uncommon to find 
standard thermometers reading anywhere from 30° to 
even 60° in error due to their misplacement. In one 
kiln which the writer examined a mercurial thermom- 
eter was placed in a recess in the front door in such 
a way that it could be read through a glass from the 
outside. The air in this kiln was supposed to be enter- 
ing the lumber at 160° F., and indeed the thermometer 
so indicated. Upon entering the kiln and hanging a 
test thermometer near the pile of lumber, where it was 
allowed to remain for fifteen minutes to come to equi- 
librium, I was surprised to find the actual temperature 
to be 230° F. ! The thermometer by which the kiln was 

265 



266 THE KIEN DRYING OF LUMBER 

operated was in a sheltered nook where it received cold 
radiation from the door and was indicating seventy 
degrees too low! 

A thermometer should be so located that it can not 
receive direct radiation either of heat from the steam 
pipes nor of cold from the lumber or from the door or 
even the roof or walls of the kiln. Moreover, it must 
not be placed in stagnant air. Too much importance 
can hardly be put on the question of the proper placing 
of the thermometers. Merely to stick a standard ther- 
mometer somewhere in the kiln and expect it to give 
you the correct temperature is as unreasonable as to 
expect an alarm clock to wake one up at the proper 
time, without setting it. It matters little what kind 
of a thermometer is used. An ordinary 50-cent ther- 
mometer may be as useful as a $10 standard if it is 
properly used. It is always wise, furthermore, to have 
more than one thermometer, and to check the two 
readings, one against the other. 

A recording thermometer, with a long, flexible tube 
connection with the bulb is one of the most useful kinds 
for a dry kiln, as a weekly record may then be kept 
continuously. Such instruments can be obtained from 
any reliable instrument maker and cost from $35 to 
$65. Mercury-filled connecting tubes have not proved 
satisfactory for dry kiln use, and the gas-filled kind is 
strongly advised. 



USEFUL INSTRUMENTS 267 

(2) Humidity. — The humidity is more difficult to 
determine than temperature, but does not require com- 
plicated instruments. The most satisfactory for the 
purpose consists of the ''Wet-and-dry-bulb Hygrom- 
eter,*' which consists simply of two glass stemmed 
thermometers hung side by side, the bulb of one being 
surrounded by a wick which dips into a vessel of water. 
Such instruments can be procured in the market or 
can be readily homemade. The essential points are 
that the wick shall be kept clean, free from grease or 
scale incrustation ; the wick must be thoroughly moist ; 
distilled water should be used in the reservoir and the 
bulb should be hung where it receives a good draught 
of air. Otherwise the same precautions which were 
given for temperature determinations apply to this 
instrument. The wet-and-dry-bulb readings are taken 
simultaneously and the humidity may then be quickly 
determined from the humidity chart given in Chapter 
XIV. As it is fully explained, further remarks are 
unnecessary here. The wet-and-dry-bulb hygrometer 
is sometimes known as a * * hygrodeik. " 

A form of this instrument adapted to the recording 
thermometer is now being manufactured by the makers 
of the recording thermometers, with the two pens 
recording on the same dial. A caution in the use of 
this recording hygrometer is necessary, first, that the 
wicks and reservoir must be so arranged as to insure 



i 



268 THE KILN DRYING OF LUMBER 

that the bulb be kept continually wet, and, second, 
that the wet bulb be placed so that it will receive a 
full current of air. 

A hand instrument inclosed in a convenient case 
having a small self-contained electric fan to produce 
the current of air over the wet bulb is on the market, 
and is a useful form of this hygrometer. 

There are a number of direct reading hygrometers. 
Of these the small dial instruments which operate by a 
fine spiral band composed of two thin layers, one of 
metal and the other of some hygroscopic material, have 
proved too perishable for dry kiln work and are not 
recommended for this purpose. Another form of hy- 
grometer in which the hand on a dial is operated by 
a number of fine hairs is more satisfactory, but rather 
too perishable for steady work. 

Disks of paper or cloth soaked in a solution of 
cobalt chloride are a very good rough indicator as to 
whether the air is very damp or dry. When dry the 
cobalt chloride turns a brilliant blue, when damp it is 
pink, the changing point being in the neighborhood of 
60 per cent, humidity. 

(3) Determination of Moisture in the Wood. — Dif- 
ferent wood users have various empirical methods of 
deciding whether wood is sufficiently dry for their use. 
Many of these are based on long and intimate personal 
experience in the qualities of the wood as affected by 



USEFUL INSTRUMENTS 269 

the moisture, and a discussion of them here would not 
be of much service, since such knowledge can he gained 
only hy observation and experience. There is, how- 
ever, a simple and accurate method which anyone can 
make use of. It consists in taking a suitable sample 
of the wood to be tested, weighing it, then drying it 
either in an oven heated to about 212° or simply laying 
it on top of a radiator until it ceases to lose weight, 
then weighing again. The loss in weight is the amount 
of moisture which the piece contained, and this loss 
divided by the dry weight times 100 is the moisture 
per cent, as scientifically used, the dry weight of the 
wood being the basis of the percentage expression. 
Sometimes the wet weight has been used in expressing 
the moisture per cent., but this is not a good value to 
use, since it is a very variable quantity, whereas the 
dry weight of the specimen is a constant. In obtaining 
the sample, the method is to cut a disc about one inch 
thick in the direction of the grain, clear across the 
center of a representative board or stick. A disc from 
the end of the board will not answer the purpose, as 
the end is apt to have dried out considerably more than 
the rest of the board. 

For the weighing of these moisture discs a small 
balance, such as is used in apothecaries' shops, is suit- 
able, with weights from 1000 grammes to one gramme, 



270 THE KILN DRYING OF LUMBER 

or 500 grammes to one centigramme, according to 
whether large or small work is to be conducted. If 
pounds are used it is convenient to have the gradua- 
tions in hundredths rather than in ounces and grains. 

If estimates have been given in per cents of the 
green weights, the values may be converted into the 
standard moisture per cents by dividing by 100-G, thus 

^ xioo 



lOO-G^ 

where G is the moisture per cent, of the green weight 
and P that of the dry. 

The moisture distribution is often important to 
know, and this may be determined by cutting an extra 
disc and subdividing this transversely into an inner 
and outer portion. 

For some purposes the degree of dryness desired 
may be approximately determined by shrinkage meas- 
urements. A sample is removed from the kiln and its 
width measured. This is then dried in an oven or 
over steam pipes and again measured. If the shrink- 
age is more than a specified amount, the material is 
not sufficiently dry. Taking the correct condition to 
be 5 per cent, of moisture, the permissible shrinkage by 
this method per inch width of plain sawed boards for 
a number of species is as follows : 



USEFUL INSTRUMENTS 
Table XV. 



271 



Species 



Ash, white 

Basswood 

Beech 

Birch, yellow 

Cherry, black 

Chestnut 

Cypress — 

Fir, Douglas 

Gum, black 

Gum, red 

Locust, black 

Mahogany, Cuban . . 
Mahogany, African. 

Maple, hard 

Oak, white (alba) . . . 

Oak, red (rubra) 

Oak, post 

Oak, swamp white . . 

Pine, longleaf 

Pine, sugar 

Pine, western white . 
Pine, western yellow 
Pine, eastern white . . 
Walnut, black 



Plain 



Inch 

.011 to .015 

.014 to .017 

.017 to .018 

.015 

.012 

.011 

.010 

.013 

.013 

.017 

.011 

.0055 

.009 

.015 

.014 to .015 

.014 

.018 

.018 

.012 

.009 

.012 

.011 

.010 

.012 



Quartered 



.009 
.011 
.009 
.013 



Inch 

.007 to 

.010 to 

.008 to 

.012 to 

.006 

.006 

.006 

.008 

.007 

.009 

.007 

.0045 

.008 

.008 

.008 to .010 

.006 to .007 

.010 

.009 

.009 

.005 

.007 

.007 

.004 

.009 



Permanent Casehardening. — This condition is 
readily determined by cutting a narrow disc across 
the board and then slotting it crosswise into a number 
of prongs as shown in Fig. 24- A, page 120. If the 
prongs do not at first bind on the saw, but subsequently 
begin to close up after drying or in a warmed room, it 
indicates that the stick was not uniformly dry, but 
contained more moisture in the center than on the 
surface. 



CHAPTER XIII 

Temperatukes and Humidities for Drying Various 
Kinds of Lumber 

As explained fully in Chapter V, any statement in 
general of suitable temperatures and humidities with- 
out qualification as to where they apply and the method 
of drying is not only misleading but meaningless. For 
drying a single stick of wood it would be feasible to 
prescribe not only the best temperatures and humidi- 
ties, but also the length of time required, starting at 
any given moisture content, but with a pile of lumber 
this is obviously impossible. It might at first thought 
be supposed that, assuming a direct circulation through 
a pile of specified size, the conditions for the entering 
air should be the same as for a single stick by itself. 
Even this, however, may not always be strictly the 
case, for the reason that in order to secure a reason- 
able rate of drying for the rest of the pile it is often 
desirable to slightly exceed the best conditions for a 
single stick in respect to the temperature and dry- 
ness of the entering air. This is done at some risk 
to lumber on the entering side of the pile, but may be 
necessary in some cases for practical operation. In 
this connection it must be borne in mind that it is not 
possible to dry an entire pile of lumber as perfectly as 
a few pieces can be dried. Some sacrifice in quality, 

272 



TEMPERATURES AND HUMIDITIES 273 

or in time must be made to quantity in commercial 
operations. 

The clearest way in which to show the drying con- 
ditions is by means of curves on cross-section paper, 
in which time in days is the horizontal distance, or 
abscissa, and the temperature, the relative humidity, 
and the moisture content of the wood expressed in per 
cent, of the dry weight, as the vertical distances or 
ordinates. These temperatures, humidities, and per- 
centages may be all expressed by a single set of figures 
on the vertical axis running from to 180 or higher, 
as the case may be, and interpreted as Degrees Fahren- 
heit, Per Cents of Relative Humidity, or Moisture Per 
Cents, according to the respective curves in question. 

The following diagrams express what have been 
found by experiment to give the best drying results 
for a number of species of wood. They must be in- 
terpreted as applicable to a pile of lumber (slant pile, 
for example, as shown in Fig. 44, page 192, in which 
the air enters the pile at one side, passes rapidly 
through the layers and leaves at the opposite side) . The 
width of the pile, unless otherwise stated, is taken as 
only four or five feet. The main curves, heavy lines, 
give the suitable temperatures and humidities of the 
air just entering the pile at the diiferent stages of the 
drying process. Curves marked T are for temperature, 
H, relative humidity, and D, dewpoint. The conditions 

of the air leaving the pile can not be stated, but will 
18 



274 THE KILN DRYING OF LUMBER 

vary with velocity of circulation and width of pile. The 
two dash curves are the approximate moisture per 
cents, the lower one being for boards on the entering 
side of the piles and the upper one for the leaving side. 
The horizontal distance between the two is the lag in 
drying, due to the size of the pile. The time of drying 
indicated by the lower curve is, therefore, the mini- 
mum. The actual time required will be in excess of 
this, according to the width of the pile and the velocity 
of the air movement. The upper dash curve (when 
shown) is for the leaving side of a slant pile four feet 
in width when the air velocity is sufficient to carry it 
through the pile in two to three seconds. 

The curves apply to rough boards one inch in thick- 
ness with stickers of the same thickness. For drying 
other thicknesses of lumber the time ordinate should 
be increased in proportion to the thickness up to three 
inches and about one and a half times the thickness for 
thicknesses over three inches. In other respects the 
curves should be unchanged. For one-half inch or less 
the time should be decreased as the square of the 
fractional part of an inch. Thus for one-half inch in 
thickness the time should be one-quarter of that in- 
dicated in the curves. Planed lumber will dry con- 
siderably faster than rough. Quarter-sawed lumber 
will generally require 25 to 50 per cent, longer to dry 
than plain sawed. 

The curves are inclusive of all conditions from green 



WOlSTUSe PEQCCMT IN SAMPLE BEl&TIVH HUMWiTY 
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276 THE KILN DRYING OF LUMBER 

to dry. In starting with partially dry lumber, look for 
the corresponding moisture per cent, on the lower 
Moisture Per Cent. Curve, and begin with conditions as 
indicated for this point. Thus, for thoroughly air- 
dried red gum (see Plate No. 1) containing 15 per 
cent, moisture, start with the conditions as given for 
the tenth day, 130° and 61 per cent, humidity, and follow 
the curve from then on as before. It is always well, 
however, to give the air-dried lumber a preliminary 
treatment of one or two days in saturated air until the 
pile is heated clear through to the required tempera- 
ture. Steaming at a slightly higher temperature for 
the first 12 hours to one day is effective in heating 
the lumber and removing temporary casehardening by 
moistening the surface. 

The Drying Curves given in Plates I to VII are not 
to be interpreted as hard-and-fast rules, but must be 
used with intelligence and understanding. The length 
of time of drying (ordinates), especially, will vary ac- 
cording to circumstances and the initial moisture con- 
tent of the wood. Take, for example, Plate I, primarily 
for one-inch green red gum. The moisture per cent, 
curves start with 110 per cent, for the green wood. If 
the lumber to be dried averages less than this, say 60 
per cent., the time of drying may be somewhat short- 
ened. In fact, the conditions may be started with those 
indicated for the third day. A great deal depends also 
upon the use to which the wood is to be put. Thus 



TEMPERATUEES AND HUMIDITIES 277 

yellow birch can be most satisfactorily dried for high- 
grade furniture by the conditions indicated in Plate 
I, but this may be too slow in many cases, and then 
Plate III may be followed, but the lumber will not be 
as free from internal stresses as when dried by the 
slower method. Again, maple may be dried by Plate 
I with the least amount of damage, but when the white 
color of the sapwood is of prime consideration it must 
be dried at a lower humidity as given in Plate II. 

Species for which a high temperature is indicated 
can almost always be dried to even better conditions by 
a lower temperature method and a longer time, but the 
reverse is not true. The idea of the curves is to indi- 
cate not a slow method of drying, but rather the upper 
limit — the maximum conditions for producing the most 
rapid drying without undue injury to the material. 

Furthermore, it is generally true that the thinner 
the material the more intensive may the drying con- 
ditions be made. For example, while one-inch black 
walnut can be successfully dried by Plate I, for two 
and a half to three-inch thickness a lower temperature 
is desired as given in Plate IV. Or, again, one-inch 
maple may be dried as shown in Plate II, but for maple 
last blocks, the conditions given in Plate IV are recom- 
mended. 

With the foregoing explanations the curves. Plates 
I to VII, are recommended for the species indicated in 
Table XVI: 



278 



THE KILN DRYING OF LUMBER 



Table XVI. — List of Species for Drying Curves 



Plate of curves 



Principal species 



Probable species * 



Remarks 



1 inch thick 



Red gum 
Black gum 
Black walnut* 



Holly 

Cherry 

Cucumber 

Mahogany* 

Beech 

Sycamore 

Hard maple 



Suitable for most me- 
dium, dense hard- 
woods, but time of 
drying will vary with 
species. Spruce for 
special uses. 



II 
1 inch thick 



Select sap hard 
maple 

Select Bass- 
wood 



Select white 

birch 
Yellow birch 
Butternut 
Aah» 



The low humidity in this 
run is required to pre- 
vent the white maple 
sap wood from turn- 
ing pink. A low tem- 
perature below the 
fiber saturation point 
is to prevent undue 
shrinkage of the heart- 
wood. 



Ill 

1 inch thick 



Yellow birch 

Ash" 

Chestnut 



Poplar 

Cottonwood 

Willow 

TuUp 

Elm 

Hackberry 

Butternut 



Suitable for many of the 
light hardwoods. 
Time of drying will 
vary according to 
hardness of the wood 
and moisture content. 



IV 

2 J^ inches thick; 
also shaped 
thick blanks 



Black walnut 

Hard maple 
shoe lasts 

Round dog- 
wood 

Round iron- 
wood 

Mahogany 

Oak( 2 "thick) 



Also 1 to 2-inch 
Persimmon 

Ligniim vitae 
and very 
hard woods 

Eucalyptus 

Hickory, locust 

Osage orange 



Suitable for irregular 
heavy blanks. By 
prolonging the time 
almost any wood can 
be successfully dried 
by this process. 



V 
1 inch thick 



Western larch 



Cypress 



For species weak in ten- 
sion across the grain. 



VI 

1 inch thick 



Western 
cedar 
Redwood 



red 



Select white 

pine 
Sugar pine 
Western yeUow 

pine 



Especially ' 'sinker ' ' 
stock or butt logs 
which often coUapse 
in drying. 



VII 
1 inch thick 



Hemlock 
Douglas fir 
Southern yel- 
low pines 
Tamarack 
Incense cedar 



Spruces^ 
Sap white pine 
Cedars 
Sugar Pine 
Western yellow 
pine 



Some of these species 
may also be dried in 
superheated steam, but 
not so eastern hem- 
lock. Sugar Pine apt 
to brown stain. 



* This column is included provisionally, with the understanding that if may not 
be strictly applicable. 

1 Where strength is of prime importance as for aeroplanes, dry mahogany and 
Black Walnut by conditions indicated by Plate IV. 

2 For special purposes where strength is vital dry by Plate I. 



TEMPERATURES AND HUMIDITIES 279 



PLATE I 



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g 

^ 


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s 


1 


IP 


i 


i 


1 


i 


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illKli 


m 

1 


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s 


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£:: + :: 




:: 












1 1 1 1 II 1 III nil llirnrT'-lli UnHL ' 1' 1 III III 

il:gi;:;-;J-;;l;;:;;;;;;;;-;;;;;;;;:;;;;;:::;:;;;;;;;;;:;:;; 



2. ^ 6 8 fo /Z fi- 16 IS la 

Drying Conditions Suitable for One Inch Red Gum, Black Gum and Black Walnut 



280 



THE KIEN DRYING OF LUMBER 



PLATE II 




Drying Conditions Suitable for One Inch Select Sap Hard Maple and 
Select Basswood. 



TEMPERATURES AND HUMIDITIES 281 



PLATE III 



J60 




Drying Conditions Suitable for One Inch Yellow-Birch, Ash and Chestnut; 
for Ordinary Purposes. 



282 



THE KILN DRYING OF LUMBER 




TEMPERATURES AND HUMIDITIES 

PLATE V 



283 




Drying Conditions Suitable for One Inch Western Larch and Cypreas. 



284 



THE KILN DRYING OF LUMBER 

PLATE VI 





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i'""i:ii_r*"' "ir " "~^ "'^ ~ ~ ~ 'A^~~ 31." " ~~ ty 





Drying Conditions Suitable for Western Red Cedar, and Redwood "Sinker!! 
Stock, One Inch Thick. 



TEMPERATURES AND HUMIDITIES 

PLATE VII 
ZOO 



285 



/6o 




Drying Conditions Suitable for Douglas Fir, Yellow Pines, Incense Cedar and 
Many Other Softwoods. 



CHAPTER Xiy 

Humidity Diagram ^ 
purpose and construction 

The purpose of the humidity diagram is to enable 
the dry-kiln operator to determine quickly the humidity 
conditions and vapor pressures, as well as the changes 
which take place with changes of temperature. The 
diagram (Plate VIII) is adapted to the direct solution 
of problems of this character without recourse to tables 
or mathematical calculations. 

The humidity diagram consists of two distinct sets 
of curves on the same sheet. One set, the convex curves, 
is for the determination of relative humidity of wet- 
and-dry-bulb hygrometer or psychrometer ; ^ the other, 
the concave curves, are derived from the vapor pres- 
sures and show the amount of moisture per cubic foot 
at different relative humidities and temperatures when 
read at the dewpoint. The latter curves, therefore, are 
independent of all variables affecting the wet-bulb read- 
ings. The short dashes show the correction (increase 

*From Forest Service Bulletin 104, by the author, 1912. 

^For a full explanation of the psychrometer, see Weather Bureau 
Bulletin No. 235, "Psychrometer Tables," and also "Smithsonian 
Meteorological Tables." 
286 



PLATE Viri. 




m- 



HUMIDITY DIAGRAM 287 

or decrease) which is necessary in the relative humidity, 
read from the convex curves, with an increase or de- 
crease from the normal barometric pressure of 30 
inches, for which the curves have been plotted. This 
correction, except for very low temperatures, is so 
small that it may usually be disregarded. 

The ordinates, or vertical distances, are relative 
humidity expressed in per cent, of saturation, from 
per cent, at the bottom to 100 per cent, at the top. The 
abscissae, or horizontal distances, are temperatures in 
degrees Fahrenheit from 30° below zero at the left to 
220° above at the right. 

EXAMPLES OF USE 

The application of the humidity diagram can best 
be understood by sample problems. These problems 
also show the wide range of conditions to which the 
diagram will apply. 

(1) To find the relative humidity hy use of wet-and- 
dry-bulh hygrometer or psychrometer. 

Place the instrument in a strong circulation of air, 
or wave it to and fro. Read the temperature of the 
dry bulb and of the wet, and subtract. Find on the 
horizontal line the temperature shown by the dry-bulb 
thermometer. Follow the vertical line from this point 
till it intersects with the convex curve marked with the 
difference between the wet and dry readings. The 



288 THE KILN DRYING OF LUMBER 

horizontal line passing through this intersection will 
give the relative humidity. 

Example: Dry bulb 70°, wet 62°, difference 8°. Find 70" on the 
horizontal line of temperature. Follow up the vertical line from 70° 
till it intersects with the convex curve marked 8°. The horizontal line 
passing through this intersection shows the relative humidity to be 
64 per cent. 

(2) To find how much water per cubic foot is con- 
tained in the air. 

Find the relative humidity as in example No. 1. 
Then the nearest concave curve gives the weight of 
water in grains per cubic foot when the air is cooled to 
the dewpoint. Using the same quantities as in example 
No. 1, this will be slightly more than 5 grains. 

(3) To find the amount of water required to satu- 
rate air at a given temperature. 

Find on the top line (100 per cent, humidity) the 
given temperature; the concave curve intersecting at 
or near this point gives the number of grains per cubic 
foot. (Interpolate, if great accuracy is desired.) 

(4) To find the dewpoint. 

Obtain the relative humidity as in example No. 1. 
Then follow up parallel to the nearest concave curve 
until the top horizontal line (indicating 100 per cent, 
relative humidity) is reached. The temperature on this 
horizontal line at the point reached will be the dewpoint. 

Example: Dry bulb 70°, wet bulb 62°. On the vertical line for 
70° find the intersection with the hygrometer (convex) curve for 8°. 
This will be found at nearly 64 per cent, relative humidity. Then 



HUMIDITY DIAGRAM 289 

follow up parallel with the vapor pressure (concave) curve marked 5 
grains to its intersection at the top of the chart with the 100 per 
cent, humidity line. This gives the dewpoint as 57°. 

(5) To find the change in the relative humidity pro- 
duced by a change in temperature. 

Example: The air at 70° F. is found to contain 64 per cent, humid- 
ity; what will be its relative humidity if heated to 150° F.? Start- 
ing from the intersection of the designated humidity and temperature 
coordinates, follow the vapor pressure curve (concave), until it inter- 
sects the 150° temperature ordinate. The horizontal line then reads 6 
per cent, relative humidity. The same operation applies to reductions in 
temperature. In the above example what is the humidity at 60°? 
Following parallel to the same curve in the opposite direction until it 
intersects the 60° ordinate gives 90 per cent.; at 57° it becomes 100 
per cent., reaching the dewpoint. 

(6) To find the amount of condensation produced hy 
lowering the temperature. 

Example: At 150° the wet bulb reads 132°. How much water would 
be condensed if the temperature were lowered to 70° ? The intersection 
of the hygrometer curve for 18° (150°-132'°) with temperature line for 
150° shows a relative humidity of 60 per cent. The vapor-pressure curve 
(concave), followed up to the 100 per cent, relative humidity line, shows 
45 grains per cubic foot at the dewpoint, which corresponds to a tem- 
perature of 130°. At 70° it is seen that the air can contain but 8 
grains per cubic foot (saturation). Consequently there will be con- 
densed 45 minus 8 or 37 grains per cubic foot of space measured at the 
dewpoint. 

(7) To find the amount of water required to produce 
saturation hy a given rise in temperature. 

Example: Take the values given in example No. 5. The air at the 
dewpoint contains slightly over 5 grains per cubic foot. At 150° it is 
capable of containing 73 grains per cubic foot. Consequently 73 minus 
5 equals 68 grains of water which can be evaporated per cubic foot of 
space at the dewpoint when the temperature is raised to 150°. But the 
latent heat necessary to produce evaporation must be supplied in addi- 
tion to the heat required to raise the air to 150°. 
19 



290 THE KILN DRYING OF LUMBB» 

(8) To find the amount of water evaporated during 
a given change of temperature and humidity. 

Example: At 70° suppose the humidity is found to be 64 per cent, 
and at 150° it is found to be 60 per cent., how much water has been 
evaporated per cubic foot of space? At 70° temperature and 64 per 
cent, humidity there are 5 grains of water present per cubic foot at the 
dewpoint (example No. 2). At 150° and 60 per cent, humidity there 
are 45 grains present. Therefore 45 minus 5 equals 40 grains of water 
which have been evaporated per cubic foot of space, figuring all volumes 
at the dew-point. 

(9) To correct readings of the hygrometer for 
changes in barometric pressure. 

A change of pressure affects the reading of the wet 
bulb. The chart applies at a barometric pressure of 30 
inches, and, except for great accuracy, no correction is 
generally necessary. 

Find the relative humidity as usual. Then look 
for the nearest barometer line (indicated by dashes). 
At the end of each barometer line will be found a frac- 
tion which represents the proportion of the relative 
humidity already found, which must be added or sub- 
tracted for a change in barometric pressure. If the 
barometer reading is less than 30 inches, add ; if greater 
than 30 inches, subtract. The figures given are for a 
change of 1 inch; for other changes use proportional 
amounts. Thus for a change of 2 inches use twice the 
indicated ratio, for half an inch use half, and so on. 

Example: Dry bulb 67°, wet bulb 51°, barometer 28 inches. The 
relative humidity is found, by the method given in example No, 1, to 

equal 30 per cent. The barometric line (dashes) gives a value of t^ H 



HUMIDITY DIAGRAM 291 

for each inch of change. Since the barometer is 2 inches below 30, 

3 fi 

multiply Yj^ H by 2, giving j^ H. The correction will therefore be 

r^ of 30, which equals 1.8. Since the barometer is below 30, this is 
to be added, giving a corrected relative humidity of 31.8 per cent. 

This has nothing to do with the vapor pressure (con- 
cave) curves, which are independent of barometric pres- 
sure, and consequently does not affect the solution of 
the previous problems. 

(10) At what temperature must the condenser he 
maintained to produce a given humidity? 

Example: Suppose the temperature in the drying room is to be kept 
at 150° F., and a humidity of 80 per cent, is desired. If the humidity is 
in excess of 80 per cent, the air must be cooled to the dewpoint corre- 
sponding to this condition (see problem No. 4), which in this case is 
141.5°. 

Hence if the condenser cools the air to this dew- 
point, the required condition is obtained when the air is 
again heated to the initial temperature. 

(11) Determination of relative humidity by the dew- 
point. 

The quantity of moisture present and relative 
humidity for any given temperature may be determined 
directly and accurately by finding the dewpoint and 
applying the concave (vapor pressure) curves. This 
does away with the necessity for the empirical convex 
curves and wet and dry bulb readings. To find the dew- 
point some form of apparatus, consisting essentially of 



292 THE KILN DRYING OF LUMBER 

a thin glass vessel containing a thermometer and a vola- 
tile liquid, such as ether, may be used. The vessel is 
gradually cooled, through the evaporation of the liquid, 
accelerated by forcing air through a tube, until a haze 
or dimness, due to condensation from the surrounding 
air, first appears upon the bright outer surface of the 
glass. The temperature at which the haze first ap- 
pears is the dewpoint. Several trials should be made 
for this temperature determination, using the average 
temperature at which the haze appears and disappears. 
To determine the relative humidity of the surround- 
ing air by means of the dewpoint thus determined, find 
the concave curve intersecting the top horizontal (100 
per cent, relative humidity) line nearest the dewpoint 
temperature. Follow parallel with this curve till it 
intersects the vertical line representing the tempera- 
ture of the surrounding air. The horizontal line pass- 
ing through this intersection will give the relative 
humidity. 

Example: Temperature of surrounding air is 80°; dewpoint is 61°; 
relative humidity is 53 per cent. 

The dewpoint determination is, however, not as con- 
venient to make as the wet-and-dry-bulb hygrometer 
readings. Therefore, the hygrometer (convex) curves 
are ordinarily more useful in determining relative 
humidities. 



HUMIDITY DIAGRAM 293 

THEORY OP HUMIDITY DIAGRAM 

Relative Humidity Curves. — The relative humidity 
or convex curves are empirical, depending upon experi- 
mental data furnished by the United States Weather 
Bureau. The curves are based on FerrePs formula : ^ 

/•=:/i-0.000367 P (i-h) (1+^-3^) 
in which 

t = temperature of the dry bulb in degrees Fahr- 

enlieit. 
ij = temperature of the wet bulb in degrees Fahr- 
enheit. 
/ = actual vapor pressure in the air in inches of 

mercury, 
/i = maximum vapor pressure at the temperature 

of wet bulb ^1. 
P = barometric pressure in inches of mercury. 
If F be the maximum vapor pressure at the dry- 
bulb temperature t, then the relative humidity is p- 

The constants for FerrePs equation were obtained 
by a great number of careful experiments conducted by 
Professors Marvin and Hazen, of the United States 
Weather Bureau. The equation applies correctly only 
when the psychrometer is subjected to a strong current 
of air having a velocity of at least 15 feet per second. 

^For this formula and for much of the tabulated data upon which 
the curves are based, the author is indebted to the publications of the 
Weather Bureau and Smithsonian Institution referred to in the second 
footnote on p. 286. 



294 THE KILN DRYING OF LUMBEK 

It is strictly applicable only below 140° F., since the 
constants of the equation have been deduced from ex- 
periments conducted only at temperatures below this. 
Experiments made by the author indicate, however, that 
the equation holds with reasonable exactness for tem- 
perature up to the boiling point, and the curves have 
therefore been extended to 220°. 

The curves as plotted are for a barometric pressure 
of 30 inches, and since P enters into the equation a cor- 
rection must be made for exactness at other pressures. 
This correction is shown by the intersecting dashes as 
a proportion of the indicated humidity, so that it may 
be made direct from the curves with little extra cal- 
culation. These correction lines were derived in the 
following manner : 

Let P and B be the variables and i, t^, /i, and F 

f 
constant. Let C be a constant factor. Since H=-jr 

H = — p Hi p (1) 

Then, 

Hi-H _ / fi - CPi _ fi - CP \ ^ F ^ C(P - Pi) 

~Ti V F F y^fi-cp f,-cp ^^^ 

TI ^ TT 

That is, other things being constant, the ratio —^ — • is 
directly proportional to the difference in pressures (P- 
Pi). Let P-Pi = l, then 

Hi - H ^ C 
H fi-CP 



HUMIDITY DIAGRAM 295 

TT TT 1 

Now assume a value for -^ — , say jqq, then 

S^p-4''' = «""=+cp. 
But P was taken as 30 in the given curves, therefore 

'-fcP.-fcP (3) 

So also for -^ — = joq' fi == g-CP, etc. 
Now, with these values for /i corresponding to J_, 

100 
2 

2QQ, etc., proportional differences in the values of H 
for a 30-inch barometer reading, the values of CP may- 
be calculated for various differences (^-i^i) and the 
values of /i found from this equation (3). The tem- 
perature f 1 is thus determined corresponding to /i and 
therefore also the temperature t. 

Hence, points may be marked on the curves corre- 
sponding to a difference in barometric pressure of 1 
inch and a proportional value of H added to or sub- 
tracted from the value derived straight from the curves. 
This proportion is given at the left-hand end of these 
barometric lines. It will be seen from equation (2) 
that this amount must be added to H for barometer 
readings less than 30 inches, and subtracted for read- 
ings above 30 inches. The amount is also seen to be 
directly proportional to the difference in pressure, so 
that for a fall of barometer of 2 inches twice the indi- 
cated amount must be added. 



296 THE KICN DRYING OF LUMBER 

Except for very low temperatures this correction is 
so small that it may usually be disregarded. 

Vapor-pressure Curves.'^ — The vapor-pressure or 
concave curves indicate the relative humidity or pro- 
portional saturation of the air which definite amounts 
of moisture, read at the dewpoint, will produce at given 
temperatures. The amounts of moisture are given on 
the curves in grains per cubic foot of air (or space) at 
the dewpoint or saturation. The exact weight of moist- 
ure present is not given, except at the dewpoint as just 
stated. 

The curves might have been drawn to show the 
exact weight of moisture present at any given tempera- 
ture and relative humidity, but by so doing their chief 
function, the ability to indicate the concurrent varia- 
tion of the moisture condition with change of tempera- 
ture, would have been lost. If the barometric pressure 
remains constant while the temperature changes, the 
vapor pressure also remains constant; but not so the 
weight of moisture per cubic foot. The relative humid- 
ity is the ratio of the actual vapor pressure to the maxi- 
mum vapor pressure ; hence by drawing these curves to 
represent vapor pressures instead of weights of vapor, 
they may be followed strictly from one temperature to 
another for determining the change which will occur 
in the relative humidity. The difference between the 

* Not to be confused with saturate vapor-pressure curves. 



HUMIDITY DIAGRAM 297 

density (weight) curves and the vapor-pressnre curves 
is comparatively small, however, within the range of 
temperature given, and the curves may be read in terms 
of grains per cubic foot at any point without great 
error. For example, take a cubic foot of saturated air 
at 70° F. and heat it to 100°. The concave curves of 
the humidity diagram show that it originally contained 
8 grains of moisture; the saturated vapor-pressure 
curve shows that the saturate vapor pressure was 0.73 
inch of mercury. In heating to 100°, the total pressure 
of air plus moisture remains unchanged, and therefore 
the vapor pressure is still 0.73 inch. The vapor, how- 
ever, has been expanded by the heat so that it now 
occupies 

<o+100_561 , „^^ , . , ^ 

^0 being the absolute temperature Fahrenheit. One 

cubic foot therefore contains — ^ = 7.57 grains. The 

relative humidity by the curves is seen to be 38 per 

cent. (:=,^'\ . Had this air been heated to 170°, 1 cubic 
V 1.927 ' 

foot would contain 6.73 grains instead of 8, and its rela- 
tive humidity would be 5^=6 per cent., which is 
shown on the curve. The denominators are pressures 
of saturated vapor (in inches of mercury) at the re- 
spective temperatures. 

Saturate Vapor-pressure Curves. — The four curves 
shown at the top of Plate VIII represent pressures of 



298 THE KILN DRYING OF LUMBER 

saturated vapor, the vertical lines (ordinates) being 
pressures in inches of mercury, and the horizontal lines 
(abscissae) being temperatures. They are in reality one 
curve drawn in four parts, in as many different scales, 
each of which is the previous scale magnified by ten. 
For example, the units on the first part of the curve (on 
the extreme right) are inches ; when the curvature de- 
creases so that this scale can not be read accurately, 
the curve is broken and the second part is drawn in 
units of 0.1 inch. Similarly the units shown on the 
third and fourth parts are, respectively, 0.01 and 0.001 
inch. 

MEASUEES AND WEIGHTS 

The following equivalents are given for convenience 
in making computations : 

9°F. = 9°C. 

1 inch = 25.4 mm. 

1 meter = 39.37 inches. 

1 grain = 0.06480 gram. 

1 gram =15.4324 grains. 

1 grain per cubic foot = 0.0022883 gram per liter. 

1 gram per liter = 436.99 grains per cubic foot. 

7000 grains = 1 pound av. = 453.59 grams. 

1 U. S. gallon water at 62° F. weighs 8.3360 lbs. av. 

1 cu. ft. water at 62° F. weighs 62.36 lbs. av.; at 39° F. (maxi- 
mum density), 62.43. 

1 cu. ft.= 7.48 U. S. gallons. 

1 U. S. gallon = 231 cubic inches = 0.1368 cu. ft. 

1 imperial gallon water at 62° F. weighs 10 lbs. av.= 277.11 
cubic inches. 

1 cu. ft.= 6.236 imperial gallons. 

1 U. S. gallon = 3.785 liters. 

1 imperial gallon = 4.541 liters. 



HUMIDITY DIAGRAM 299 

1 cu. ft. = 28.317 liters. 

1 cu. ft. of dry air at 32° F. weighs .08071 lb. 

1 cu. meter of dry air at 0° C. weighs 1.293 kg. 

Coefficient of expansion of air= jo] of volume at 32° F. per 

degree F. 
Eatio weight saturated vapor to weight dry air at same pres- 
sure and temperature is nearly constant =: 0.6225. 
VP = ilT, where V = volume of gas in cubic feet. 

P = absolute pressure in lbs. per cubic foot. 
T = absolute temper ature = t + 46 Iwhere t is 

temperature Fahrenheit. 
R = 53.37 for air. 
Specific heat of air at constant pressure K = 0.2375 B. t. u. per lb. 
Kp — Kv = (R-i- 778) where Kyis specific heat at constant 
volume = 0.1689. 

Latent heat of ice = 142 B.t.u. per lb. 

Specific heat of ice = 0.504 B.t.u. per lb. 

Specific heat of dry wood = 0.33 B.t.u. per lb. 

Latent heat of steam at 212° =970 B.t.u. per lb. 

Latent heat of steam at 307° (60 lbs. gauge) = 904 B.t.u. per lb. 



APPENDIX 

Special Woods foe War Uses 
species most suitable fob exacting requirements 

The exigencies brought about by the war have made 
it absolutely necessary to kiln dry much material which 
heretofore it has been customary to air dry for the same 
purposes, the normal supply of air-dry stock being al- 
together inadequate and the time required too short in 
which to air dry new material. This, however, is by 
no means an alarming nor a regrettable situation, since 
the same material can be kiln dried directly from the 
saw in equally as good condition and in many cases 
much better condition altogether as to strength, tough- 
ness, and freedom from defects. Material requiring a 
year to air dry can be kiln dried in three weeks and 
that requiring from 3 to 5 years to thoroughly season 
can be dried in 3 to 5 months. Strength tests are being 
made by the Groverimient which prove that the properly 
kiln-dried material is fully equal to the air-dried in 
every respect. Of course, it is understood that suit- 
able methods must be used, and the operation intelli- 
gently conducted, otherwise serious injury is likely to 
result. In many cases a strong prejudice has arisen 
against kiln-dried wood owing to the bad methods 
which have been used and consequent injurious results 
obtained. 

300 



APPENDIX 301 

Not only must green wood be resorted to but sub- 
stitute species are being urgently sought after. 

Of the species called for by the war conditions which 
must be dried from the green condition and their uses 
the following are of the greatest importance: 

(1) Airplanes. — ^Sitka spruce (density should be at 
least 24 pounds per cubic foot for the dry (8 per cent.) 
wood, and having not less than 10 rings per inch) is 
one of the best woods when strength, stiifness, and 
lightness are required. It is used for the wing beams 
and struts and is in great demand in 2- and 3-inch thick- 
nesses. "White ash is also largely used for longerons in 
inch and a half thicknesses. The northern-grown ash 
of fair density (at least 35.6 pounds per cubic foot for 
the 8 per cent, dry wood) is considered the best. Some 
sugar pine is often used in the wing ribs. These should 
be dried to about 8 per cent, moisture, or for use on the 
sea coast probably 12 per cent, is ample. It is unsafe 
to dry too low, as brittleness must be avoided. Some 
maple and birch are used for certain parts. 

For propellers various species are used, chiefly Hon- 
duras and Nicaragua mahogany, black walnut, white 
oak, and yellow poplar (Tulip). Mahogany and walnut 
are used almost exclusively abroad. 

Careful attention must be given to density, freedom 
from cross-grain and internal stresses (caseharden- 
ing). A dryness of about 8 per cent, appears to be 



302 THE KILN DRYING OF LUMBER 

suitable for gluing. For this purpose ** quarter- 
sawed" (radial cut) material should be given prefer- 
ence as far as possible on account of small shrinkage 
and balanced internal stresses, but it is doubtful 
whether large enough pieces are now obtainable from 
black walnut for quarter sawing. 

(2) Gun Stocks J — ^Black walnnit is the universal 
wood for this purpose. It is usually cut in 2i/^ or 2% 
inch planks and the stocks are sawed to rough pattern 
in the green. These are at once steamed at about 140° 
for three days to darken the color of the sapwood, the 
ends are coated, and then they are shipped to the gun 
makers. Either quarter or flat grain is acceptable, 
but it must be straight and free from all twist, worm 
holes, checks, knots, or evidences of decay. The best 
quality material for this purpose has a density of the 
kiln-dry wood at 7^ per cent, moisture of 44.5 pounds 
per cubic foot, but the material varies from 31 to 51 
pounds. 

Yellow birch {Betula lutea) has proved to be a very 
good substitute. It is, as a rule, slightly heavier and 
more inclined to warp, but much more easily dried. 
The best quality birch for the purpose weighs 43 pounds 
per cubic foot when kiln dry at 1^2 P^r cent, moisture. 
The average shrinkages from green to perfectly dry 
condition in volume are 11.3 per cent, for walnut and 
16.8 per cent, for birch, based on the volume when 
green. 



APPENDIX 303 

Gun stocks should be dried to an average of 7i/^ 
per cent, moisture. 

(3) Wagons. — Escort wagons are made almost en- 
tirely of oak and hickory. Some of the parts are 5 
inches thick in the rough. Felloes are 3^ inches square 
in cross section. The former require 4 to 5 months 
to kiln dry without injury and the felloes about 3 
months. Dry wagon stock to 8 or 10 per cent. 

Other species used in wagons are ash, rock elm, 
maple, and birch ; also yellow pine, sap gum, and other 
soft woods for the boxes. 

(4) Shuttles. — The increase in demand for cloth has 
called for such an increase in shuttles that green wood 
must be resorted to. Dogwood and persimmon are the 
only two species universally used for shuttles in this 
country. In Europe boxwood is considered the finest 
material. These woods can be successfully kiln-dried, 
but a slow process is necessary at low temperature. 
About 5 per cent, moisture is a suitable condition to 
which to be dried. Applewood is recommended by some 
manufacturers as a substitute. Hop hornbeam (Ostrya 
virginiana) appears to be equal to dogwood in every 
respect, but is not abundant. It is suggested that some 
species of eucalyptus growing in California {E. sider- 
oxylon, or E. rostrata) might make good substitutes, as 
also Jarrah {E. gomphocephala) and other species 
which, however, are not available in this country. 

(5) Wood Steamships. — ^While ships are often built 



304 THE KILN DRYING OF LUMBER 

entirely of green wood it is conceded that air-dried ma- 
terial will make the most durable ships. Furthermore, 
a ship built of partially dried wood will tighten and 
stiffen up when placed in the water. The ''standard" 
ships are made almost entirely of southern yellow pine 
or of Douglas fir.^ Very little else is used. Large 
sized material is called for, much of the yellow pine 
being 14" X 14" and 30 to 50 feet long, and running as 
large as 16" X 24" for some parts. In Douglas fir con- 
siderable material 20" X 20" and 20 to 50 feet long is 
required. 

Some white oak is used, in such places as the rudder 
post, shaft log, stem post, keel shoe, and at the turn of 
the bilge. Knees are usually of live oak, white oak, 
tamarack, spruce, cypress, or white cedar, and are cut 
from the crook in the log near a root. Treenails are 
generally made of white oak or black locust. Osage 
orange should be equally good for this purpose. 

In drying, the ceiling appears to be the most impor- 
tant, and it is probable that a dryness of 15 or even 20 
per cent, is ample, as this will allow a slight swelling 
when the ship is placed in the water. 

Instructions already given in Table XVI, for drying 
the various species, should be modified, as noted in the 
footnotes on page 278, for all special uses where 
strength and toughness are of vital importance. 

* For information and specifications address " Emergency Fleet 
Corporation, U. S. Shipping Board, Washington, D. C." 



INDEX 



Absorption of moisture by cement, 

etc., 95 
Accessories, 76 
Acids, 20 
Advantages to be gained by kiln 

drying, 8 
Aeroplanes, 301 
Air drying, 3 
"Air dry," 3 
Air seasoning, 4 
" Air-dried " wood, 5 
Air drying versus kiln drying, 24 
injurious, 24 
losses, 54 
Air movement, 157 
cause of, 167 
how indicated, 168 
Air flow, path of least resistance, 

176 
Albumenoids, 20 
Amount of lumber kiln dried, 7 

heating pipes, 63 
Ammonia, fuming with, 190 
Analysis of heat quantities, 235 
Annual saving by kiln drying, 8 
Anatomy of wood, 10 
" Annual rings " or " grain," 10 
Annual rings, lacking in tropical 

woods, 18 
" Angiosperms," 12 
Analysis for mineral content, 22 
Application of analysis to water 
spray or condenser 
kilns, 246 
to ventilating kilns, 249 
Appendix, special woods for war 
uses, 300 
20 



Apparatus, 6 

Applewood for shuttles, 303 

Art of drying, 137 

United States in lead, 37 
history and development 
of, 37 
Arrangement of pipes, 58 

of the kilns, layout of plant, 
100 
Ash for aeroplanes, 301 
content of wood, 22 
Automatic sprinklers for fire, 87 

Bamboo, 18 

Babcock's formula, 66 

Baffle plates, 193 

Birch for gunstocks, 302 

Black locust for treenails, 304 

Black walnut for airplanes, 301 

for gunstocks, 302 
Blower kilns, 41 
Boiling kiln, 46 

process, 200 

in water, 110 

not injurious, 140 

explanation, 218 
Boiler, horsepower required, 63 
" Bordered pits," 13, 14 
structure of, 17 
Broad-leaved trees, 12 
Brick kilns, for drying brick, 35 
Brittleness, 182, 187 
Building lumber, amount used, 7 
Buoyancy of wood, 19 
Building materials, 91 
Butt portions of trees, 109 
Buried wood, 109 

305 



306 



INDEX 



Causes of various effects, 184 

of fire, 87 
Calculations for heating capacity, 
61 
for Table IX, 254 
"Cambium," 10 
Canvas curtains, 80 
Casehardening, 103 

explanation, 114, 115 
determination of tests for, 

271 
problem in drying, 185 
removal of, 120 
relieved by steaming, 120, 
122, 213 
Casehardened wood, stresses in, 

118 
Cell walls, 3, 13 

composition, 19 
Cells, 10, 11 
Cellulose 20 

composition of, 20 
cotton, 20 
Checking, 184 
Chemical composition of wood 

substance, 20 
Change of color, 183 
Charge kiln, 31 
Circulation, 137 
sluggish, 138 
reversible, 148 
and method of piling, 154 
importance of, 155, 220 
indicated by temperatures, 

169 
baffled in flat pile, 169 
actually observed diagrams, 

170 
in edge-stacked pile, 170 
in inclined pile wrong, 171 
in inclined pile correct, 172 
opposed by condensers, 173 



Circulation, downward in venti- 
lated kiln, 173 
self -regulatory, 176 
opposed by forced draught, 
177 
Classification of methods of dry- 
ing, 30 
for lumber, drying kilns, 35 
Cobalt chloride, indicator of 

moisture, 114, 268 
Coils of pipe, continuous, 56 
Collapse in wood, 117 

of the cells, 183, 185 
Colloidal substance, wood, 20 
Cohesion between the fibers, 182 
Cold pile of lumber a condenser, 

163 
Common practices in drying, 23 
Comparison of efficiencies, 247 

between forms of piles, 145 
Composition of cellulose, 20 
of starch, 20 
of sugar, 20 
Compartment kiln, 31 

vs. progressive kilns, 102 
Conclusion as to drying in vapor 

and in air, 230 
Conduction of water up the tree, 

17 
Condenser kilns, 43 

application of analysis 
to, 246 
Condensers, 43 
Condensing pipes, 74 
Condenser, quantity of water, 75 

heat extracted by, 75 
Condensation, how eliminated, 199 
Condensation and drip, 148 
Conduction of heat, 155 
Concave surface, 120 
Conifers, 12 

Construction materials for kilns, 
196 



INDEX 



307 



Construction and cost of dry kilns, 

88 
Consumption of lumber by wood- 
using factories, exports, 
railroads, building, 7 

of wood per capita, 2 

of lumber softwoods, 7 

of lumber hardwoods, 7 
Convex surface, 120 
Convection, 154 
Cooperage, 12 

Cotton, is pure cellulose, 20 
Coatings for wood, 84, 134 
Cost and construction of dry 
kilns, 88 

example, 96 

of operation, 98 

of handling, 99 
Covering for pipes, 57 
Creosote, 134 
"Cross piling," 82 
Crucial point in the drying, 152 
Cupping when resawed, 119 

of slash cut boards, 182 
Curtain, canvas, 80 
Cutting season, 108 
Curly grain, 124 
Cycle in drying operation, 237 
Coefficient of expansion of air, 299 

Darkening of sapwood, 29 

wood by fuming with am- 
monia, 190 
Decay of wood, 4, 108 
Definite direction process, 144 
Density of wood, 19 
Density of air and vapor in- 
creased by spontaneous cooling, 
253 
Destruction of forests, 1 
Determination of temperature, 
265 
of humidity, 267 



Determination of moisture in wood, 

268 

of moisture per cent., 269 

of casehardening, 271 
Dewpoint, 222, 224 
Dewpoint method of drying, 198 
Dicotyledons, 19 
Diffusion process in drying, 144 

of gases, 155 
Discoloring, 185 
Distillation of wood in kiln, 188, 

213 
Doors of dry kiln, 76, 79 
Door carrier, 79 
Dogwood for shuttles, 303 
Douglas fir for ships, 394 
Downward circulation, 156 

in ventilated kiln, 173 
Drip and condensation, 148 
Drip, how eliminated, 199 
Drop in temperature, 145 
Dryers, other kinds, 32 

veneer, 32 

varnish, 34 
Drying (see methods of drying) 

process of, 141, 235 

curve, form of, 149 

curves Plates I to VII, 279- 
285 

curves list of species, table, 
278 

crucial point in, 152 

affected by thickness, 274 

at pressures other than at- 
mospheric, 200 

elementary principles of, 216 

by expa..sion of air, 29 

from inside outwardly, 207, 
210 

problems, 180 

paper, 138 

from opposite surfaces, 161 

rate, how affected, 153 



308 



INDEX 



Drying by girdling, 26 
by leaf, 26 
repeated, 188 
in superheated steam, 47, 

200-204 
a single stick, 155 
by radiation, 205 
under vacuum, 201 
veneer, 138 

various kinds of lumber, 272 
Dry in the air, 3 
Dryness, determination of, 270 
Dry kilns, heating in, 56 

building materials, 30 
compartment or charge, 

31 
for bricks, 35 
blower, 41 
construction and cost of, 

88 
capacity, 68 

radiation from walls, 71 
internal circulation in, 

42 
condensing, 43 
Tiemann, 44 

steam spray-condenser, 45 
superheated steam, 46 
boiling, 46 

humidity regulated, 191 
moist air, 38 
ventilated, 38 
forced draught, 41 
high velocity, low super- 
heat method ( Tie- 
mann, Betts), 48 
doors, 79 
a machine for drying 

lumber, 143 
earliest, 36 
classified, 35 
progressive, 31 
patents, 31 



Durability not 
steaming, 211 



increased by 



Earliest kilns, 306 
Edge piling and stacking, 78, 160 
Effect of salt water, 26 
soaking, table, 27 
internal heat, 114 
methods of drying upon 

strength, table, 256 
methods of drying upon hy- 

groscopicity, table, 263 
which result from drying, 184 
of preliminary steaming, 208 
EfiBcieney of a dry kiln, 88 
of operation, 240 
of operation, conclusions, 246 
Efiiciencies, comparison of, 247 
Elementary principles of drying, 

216 
" Endogens," 18 
End piling, 82 
End coating, 84 
End drying, 112 

Estimate of saving by kiln dry- 
ing, 51 
Erosion of soil, 1 
Evaporators, 34 

Evaporative capacity of super- 
heated steam, 201 
Evaporation requires heat, 216 
rate controlled by humidity, 

221 
by vacuum alone is a fallacy, 

217 
proportional to heat supplied, 

218 
main things which influence, 

220 
in absence of air, 225 
when air is present, 229 
from a free surface of water, 
227 



INDEX 



309 



Evaporation from capillary spaces, 
183 

equation of, 229 

theoretical discussion, 225 
Examples of heating capacities 

of kilns, 59 
Exhaust steam, low pressure, 58 
"Exogens," 18 
"Explosion," 186 
Exports of lumber, 7 
Expansion of air, drying by, 29 

Ferrel's formula, 293 

Fiber saturation point, 4, 105 

table, 104 
Firs, structure of 16 
Fire protection, 76, 85 

cause of, 87 

sprinklers 87 
Fixation of gums, resins, etc., 210 
Flat crosswise piling, 159 
Flow of steam in pounds per min- 
ute, table, 66 
Forced draught, 176 

kiln, 41 
Form of the drying cxxrve, 149 
Forms of piling, 8i0 
Forestry, 2 

Forest Products Laboratory, 49 
Forests, destruction of, 1 
" Free water," 103 

internal, 152 
Frozen lumber, 163 
Fumes from lumber in kiln, 76 
Fuming with ammonia, 190 
Fungous growth and rot, 108, 211 

Galvanized iron pipe, 57 
German methods, 38 
General layout of plant, 100 
■Generalization of theoretical heat 

relations, 244 
Girdling trees, 26, 108 



Grain, curly, 124 

interlocking, 124 

spiral, 124 
Green wood, 103 
Gums, 20 

Gunstocks, 83, 302 
Gymnosperms, 12 

structure of, 16 

Habitability of the land due to 

forests, 2 
Hardwoods, 12 
Hartig's investigations, 106 
Hardening of gums, resins, etc., 

211 
Hausbrand's tables, 244 
H'eartwood, 11 

Heat and cold aflFect transfusion 
of moisture, 112 
consumption, 71 
conductivity, table, 95 
of adsorption, 152, 184 
conduction, 155 
of affinity, 103 
efficiency, 88 
influences rate of transfusion, 

139 
extracted by condensing 

water, 75 
losses, tests, 94 
produces evaporation, 139, 

216 
quantities, 62 
quantities, analysis of, 235 

example, 242 
radiation, 64 

relations, generalization, 244 
required to evaporate one 

poimd, table, 242 
transmission, 93 
Heating capacities, examples of, 
69 
capacity, calculations for, 61 



310 



INDEX 



Heating capacities of air and vapor 
in mixture, 232 
of air and vapor, sepa- 
rately, 233 
in dry kilns, 56 
surface, 59 

wood, three ways, 220 
Height of pile, 82 
Hickory for war wagons, 302 
High velocity low superheat 
method (Tiemann and Betts), 
48, 202, 203 
High temperature, effects of, 189 
Highest possible theoretical effi- 
ciency, 243 
Holders for staves, etc., 79 
" Hollowhorning," 117 
Hot plates, drying, 155 
Honeycombing, 117 184 
Hot water, 110 

How lumber should be piled, 142 
How wood dries, 103 
Humidity, 137, 138 

control in Tiemann kiln, 195 
determination of, 267 
dependent on circulation, 154 
diagram, purpose and con- 
struction, 286 
examples of use, 287 
theory of, 293 
explanation, 218 
regulated dry kiln, 191 
regulation, 224 
Humidities and temperatures for 

drying lumber, 272 
Hussey dry kiln door carrier, 80 
Hydrolysis of cell walls, 213 
Hygrodeik, 267 

Hygrometers, other kinds, 268 
recording, 267 
wet and dry bulbs, 267 
Hygrometry, principles of, 222 



Hygroscopicity, 182, 228 
of wood, hypothesis, 19 

effect of methods of dry- 
ing upon, 256 
Hygroscopic moisture, 103 

Ice to be melted, 73 

Importance of circulation, 155, 

220 
Inclined piling, 83, 159 
Inclined rails, 77 
Increase in density produced by 

spontaneous cooling, table, 253 
Insulation, 92 
Insulators, 93 
Instruments useful in dry kiln 

work, 265 
Internal circulation kilns, 42 
Internal free water, 152 
Internal heat, effect of, 114 
Internal pressures in living tree, 

107 
Internal stresses, 116 
" Interlocking grain," 124 

Janka's experiments, 25, 106 
Japanese wooden articles, 110 

Keynote to successful drying, 156 

Kiln capacity, 68 

drying advantages in, 8 
annual saving, 8 
vs. air drying, 24 
saving by, estimate, 51 
of lumber, 1 
objects of, 5 
principles of, 137 

Kiln dry wood, specifications, 8 

Kinds of cells, 11 

Knees for ships, 304 

Lag in drying due to size of pile, 

147, 274 
Leaf drying, 26 



INDEX 



311 



Leaf, seasoning by, 108 

Leaching, 109 

Lignin, 20 

Lignocellulose, 20 

Limiting values of heat efficiency, 

243 
Literature on drying, 89 
List of species for drying curves, 

table, 278 
Living tree, 3 

Live steam, explanation, 219 
Losses in air drying, 8, 54 

in warping, checking, case- 
hardening and honeycomb- 
ing, 5 
Long seasoned timber, 136 
Low pressure exhaust steam, 58 
Lumber cut in United States 1913, 

1915, 7 
Latent heat of steam, 299 
of ice, 299 

Machine for drying lumber, 143 
Maximum possible theoretical 
heat efficiency, 243 
possible theoretical heat ef- 
ficiency, table, 245 
Mahogany for airplanes, 301 
Measures and weights, 298 
Medullary rays or "silver grain," 

11,13 
Method of drying (see drying) 
direct radiation, 205 
high velocity ( Tiemann 

and Betts), 202, 203 
quickest, 6 
steam under pressure, 

208 
reversible circulation 

(Tiemann), 204 
repeated heating and 
cooling, 205 
Methods of testing wood, 265 



Method used in calculating Table 

IX, 254 
Microscope, 10 

Microscopic, section of pine and 
of oak, 19 
views, 12 
Mineral content of wood, table, 21 
" Middle lamella," 13 
Minimum volume of air to evapo- 
rate one pound, table, 245 
Mixture of air and vapor, 236 
Moisture absorption of walls, 94 
Moisture in cell walls, 19 
Moisture disks, 269 

per cent., how determined, 

269 
in wood, determination of, 

268 
and season of felling, 106 
Mold, 186 

Monocotyledons, 18 
Motion of air, cause of, 167 
how indicated, 168 

Natural circulation is downward, 

156 
Needle-leaved trees, 12 
Non-mineral elements, 22 
Nordlinger's investigations, 106 

Oak, microscopic section of, 19 
structure of, 16 
for war wagons, 303 
Objects of drying wood, 1, 4 
Objects of kiln drying, 5 
Observed circulation, 110 
Oils, volatile and essential, 20 
" Opening the pores " a misnomer, 

212 
Operative efficiency, 88 
Operation of Tiemann kiln, 197 
Osage orange for treenails, 304 
Other kinds of dryers, 32 



312 



INDEX 



Palm trees, 18 

Patents, 6 

Path of least resistance, 137 

Paraffin, 134 

for coating, 85 
Permanent casehardening, de- 
termination of, 271 
Persimmon for shuttles, 303 
Pile, comparison between, 145 

internal portions of, 143 

interior not heated by radia- 
tion, 142 

size of, 142 

shape of, 142 

with reference to air move- 
ment, 157 

width of, 144 
Piling of lumber, 142 

best arrangement, 160 

endwise, 82 

edgewise, 160 

crosswise, 82 

inclined, 83, 159 

gunstock blanks, 83 

flat crosswise, 159 

forms of, 80 

methods contrasted, 157 

shingles, staves, laths, and 
shoe lasts, 84 
Piping, 58 

Pipes, condensing, 74 
Pine, microscopic section of, 19 
Plant, layout, 100 
Plasticity of wood, 182 
Porous woods, 12 
" PowelUzing process," 133 
Prejudice against kiln-dried wood, 

300 
Preliminary steaming, 29, 207 
effect of, 208 

treatments to drying, 25 
Pressures of air and vapor, 223 



Pressures occurring within the 

tree, 107 
Present practice, 23 
Principal groups of woods, 11 
Principles of kiln drying, 137 

of hygrometry, 222 

of the condenser, 224 

governing the drying opera- 
tions, 140 
Process of drying, 141 
Process of transfusion, 112 
Progressive nature of drying, 144 

kilns vs. compartment, 102 

kiln, 31 
Progression of drying through the 

pile, 177 
Propellers for airplanes, 301 
Properties of wood which affect 

drying, 180 
Protoplasm, 11 

Quantity kiln dried, 7 
of pipe, 58 
of water, 75 

of air and of steam required 
to evaporate one pound of 
water, 234 
Quickest method of drying, 6, 200 

Kadiation of heat, formula, 64 

from kiln walls, 71, 92 
Rate of drying changes, at fiber 
saturation point, 151 
how affected, 153 
transfusion of moisture in 

wood, 181 
drying, 11 

evaporation controlled by 
humidity, 221 
Railroads, wood consumed by, 7 
Rails, 76 



INDEX 



313 



Rapid drying by superheated 

steam, 49 
Rattan, 18 

Recording thermometers, 266 
Red gum, cells, 15 
Reducing valve for steam heat- 
ing, 61 
Reduction of hygroscopicity, 189 

in internal stresses, 136 
Refrigerating capacity of frozen 

lumber, 163 
Regulation of heat, 61 
Relative humidity (see humidity) 
and mois.ture in wood, 

103 
and percentage retained, 

104 
of superheated steam, 
228 
Removal of casehardening, 120, 
122, 213 
dryinng, 206 
Repeated heating and cooling, 

drying, 206 
Repeated shrinkage and swelling, 
135 
soaking and drying, 136 
Resonance properties of soft 

woods, 16 
Resin ducts, 16 
Resins, 20 
Reversible circulation method of 

drying, 148, 204 
Reversal of stresses, 123 
Rot and fungous growth, 108, 211 

Salt water, effect of, 26 

Sap wood, 10 

Sap, 107 

Saturated vapor, 223 

pressure curves, 297 
Saving by kiln drying, 61 



Season of felling and moisture, 
106 

and water, 107 
Seasoning in water, 109 
Ships, 304 
Shrinkage, 103, 125 

and age, 136 

by corrugating, 128 

and density, 127 

diflFerent in different direc- 
tions, 181 

of disks, 126 

of eucalyptus, 132 

not eliminated, 133 

of fiat sawed lumber, 126 

heart and sapwood, 128 

longitudinally, 126 

and moisture relation, 127 

permissible in boards, table, 
270 

prevention, 134 

of quarter-sawed lumber, 126 

and swelling repeated indefi- 
nitely, 135 

takeup, edge stacking trucks, 
78 

table VII, 127, 129 

varies with conditions of 
drying, 128 

variable, 182 
Short circuit the piles, 157 
Shuttles, 303 

Sitka spruce for aeroplanes, 301 
Size of pipe main, 68 
Size of pile, 143 
Sluggish circulation, 138 
Soft woods, structure of, 12, 16 
Soaking in soft and also in salt 
water, 25 

in water, 109 
Sounding boards, 16 
Special woods for war uses, 300 



314 



INDEX 



Specifications for kiln dry wood, 

8 
Special problems in drying, 180 
Specific gravity of wood sub- 
stance, 19 
Specific heat of wood, 206, 299 

air at constant pressure, 

234, 238, 299 
superheated vapor, 238 
water vapor, 233 
ice, 299 
Spiral grain, 124 
Spontaneous combustion, 86 

cooling increases density of 
air, 254 
Spontaneous cooling causes air to 
descend, 156 
vaporization, 209 
Sprays in kiln, 193 
Spray chambers, 193 
Stacking (see piling) 
Stagnation, 173 
Starch, composition of, 20 
Steam consiunption, 63 
main, 56 
pipes, 56 
quantity and condensed per 

hour, 65 
sprays, 45 
Steaming alleged to "open the 
pores," 212 
to darken color, 188 
effect on rate of drying, 214 
effect on shrinkage, 214 
experiment on basswood, 210 
preliminary treatments of, 

29 
to prevent mold, 187 
under pressure, 207 
to remove casehardening, 122, 

213 
not injurious, 140 



Strength and temperature, 188 
effect of methods of drying 
upon, 256 
Stickers, 81 

Structure and properties of wood, 
10 
of bordered pits, 17 
of firs and gymnosperms or 

softwoods, 16 
of oak, 16 
of wood, 10 
Stresses, internal, 116 

in casehardened wood, 118 
Sugar cane, 18 
Sugar, composition of, 20 
Sugar treatment, 133 
Superheated steam, drying in, 47, 
200 
difficulties with, 231 
economical of heat, 241 
evaporative capacity of, 

201 
explanation, 219 
kilns, 46 
improved method, 202, 

203 
objections to, 201 
relative humidity of, 228 
vapor equivalent to moist 
air, 233 
"Sweating," 149 
Swelling of wood, 134 

Teakwood, 26 
Teak forests, 108 
Temperature, 23, 137 

affects physical character, 
139 

affects strength, 140 

control, 198 

to color dry wood, 189 

determination of, 265 






INDEX 



315 



Temperature of dry lumber, 166 
dependent on circulation, 154 
indicates circulation, 169 
of lumber in superheated 

steam, 202 
not injurious to dry wood, 

140 
of wet lumber, 165 
of wood in drying, example, 

166 
variation in pile, 169 
Temperatures in practice, 6 
Temperatures and humidities for 

drying lumber, 272 
Tendency of air to descend 

through a pile of lumber, 252 
Tests for casehardening, 118, 271 
for internal drying, 119 
for moisture, 268 
for kiln dry wood, 112 
Testing wood, methods, 265 
Theoretical analysis of heat quan- 
tities, 235 
considerations and calcula- 
tions, 216 
discussion of evaporation, 225 
Theory of transfusion, 112 
Thermal efficiency, 247, 249, 250 

expansion of wood, 126 
Thermometers, proper placing, 
266 
recording, 266 
Thickness of the cell walls, 13 
Thickness of lumber aifects dry- 
ing, 274 
Three factors of prime impor- 
tance, 137 
Time of drying and size of pile, 

147, 178 
Tops of the trees and water con- 
tent, 109 
Torus, of bordered pit, 17 



Total cut of lumber in United 
States, 1913, 7 

Total heat of water vapor, 245 

Transfusion from hot to cold, ]83 

Tracheids, 16 

Transpiration current, 106 

Transfusion from center to sur- 
face, example, 113 

Trees, internal pressure in, 107 
moisture in, 106 

Treenails, 304 

Trucks for dry kilns, 76, 77 

Tulip or yellow poplar, structure, 
12 

Tyloses, 12 

Types of cells, 12 

Vacillating currents, 179 

Vacuum process, 200 

economical of heat, 241 

Vapor and air in mixture, 236 

Varnish, 134 

Varnish dryer, 34 

Vapor pressure curves, explana- 
tion, 296 

Ventilated kilns, 38, 39 

Vermorel nozzle, 194 

Veneer drier, 32 

Vessels, 12, 13 

Volume of air required to evapo- 
rate one pound of ice, 164 

Volume of air required to evapo- 
rate one pound of water at dif- 
ferent pressures, table, 242 

Wagons, 303 
WaUs of the cell, 3, 13 
Warped direction of fibers, 182 
Warping, 103, 123, 184 
" Washboarding," 128, 184 
Water spray humidity regulated 
kiln (Tiemann), 44, 191 



316 



INDEX 



Water spray kiln application of 

analysis to, 246 
Water sprays, 194 
Water in wood, 103 
Water seasoning, 109 
War uses of wood, 300 
Weakening effect of processes of 

drying white ash, 

table, 259 
of processes of drying 

loblolly pine, 260 
of processes of drying red 

oak, 261 
Weights and measures, 298 
Wet wood, 3 

Wet and dry bulb hygrometer, 267 
Width of pile, 144 



Winter cutting, 108 

Western red cedar behavior in 
superheated steam, 202 

Wood a colloidal substance, 20 

Wood fibers in pulp industry, 18 

Wood fibers, 15 

Wood substance, chemical compo- 
sition of, 20 

Wood-using factories, 7 

" Working " of wood, 5, 133 
reduced, 183 

Wrought iron pipe, 57 

Yellow poplar or tulip, structure 

of, 12 
Yellow pine for ships, 304 



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